i've been having this problem for months now..and i have no idea what to do..

Error code: 0x 81000303

According to MSDN Library, your pssword is invalid.

There are 2 possibilites here:
1. Someone has changed your password;
2. There is something wrong with your Messenger.
    This might help, but I don't guaranteed it.

Error codes courtesy of msgrua.h:
MSGR_E_CONNECT    = 0x81000300 + 0x1,
MSGR_E_INVALID_SERVER_NAME    = 0x81000300 + 0x2,
MSGR_E_INVALID_PASSWORD    = 0x81000300 + 0x3,
MSGR_E_ALREADY_LOGGED_ON    = 0x81000300 + 0x4,
MSGR_E_SERVER_VERSION    = 0x81000300 + 0x5,
MSGR_E_LOGON_TIMEOUT    = 0x81000300 + 0x6,
MSGR_E_LIST_FULL    = 0x81000300 + 0x7,
MSGR_E_AI_REJECT    = 0x81000300 + 0x8,
MSGR_E_AI_REJECT_NOT_INST    = 0x81000300 + 0x9,
MSGR_E_USER_NOT_FOUND    = 0x81000300 + 0xa,
MSGR_E_ALREADY_IN_LIST    = 0x81000300 + 0xb,
MSGR_E_DISCONNECTED    = 0x81000300 + 0xc,
MSGR_E_UNEXPECTED    = 0x81000300 + 0xd,
MSGR_E_SERVER_TOO_BUSY    = 0x81000300 + 0xe,
MSGR_E_INVALID_AUTH_PACKAGES    = 0x81000300 + 0xf,
MSGR_E_NEWER_CLIENT_AVAILABLE    = 0x81000300 + 0x10,
MSGR_E_AI_TIMEOUT    = 0x81000300 + 0x11,
MSGR_E_CANCEL    = 0x81000300 + 0x12,
MSGR_E_TOO_MANY_MATCHES    = 0x81000300 + 0x13,
MSGR_E_SERVER_UNAVAILABLE    = 0x81000300 + 0x14,
MSGR_E_LOGON_UI_ACTIVE    = 0x81000300 + 0x15,
MSGR_E_OPTION_UI_ACTIVE    = 0x81000300 + 0x16,
MSGR_E_CONTACT_UI_ACTIVE    = 0x81000300 + 0x17,
MSGR_E_PRIMARY_SERVICE_NOT_LOGGED_ON    = 0x81000300 + 0x18,
MSGR_E_LOGGED_ON    = 0x81000300 + 0x19,
MSGR_E_CONNECT_PROXY    = 0x81000300 + 0x1a,
MSGR_E_PROXY_AUTH    = 0x81000300 + 0x1b,
MSGR_E_PROXY_AUTH_TYPE    = 0x81000300 + 0x1c,
MSGR_E_INVALID_PROXY_NAME    = 0x81000300 + 0x1d,
MSGR_E_NOT_LOGGED_ON    = 0x81000300 + 0x1e,
MSGR_E_NOT_PRIMARY_SERVICE    = 0x81000300 + 0x20,
MSGR_E_TOO_MANY_SESSIONS    = 0x81000300 + 0x21,
MSGR_E_TOO_MANY_MESSAGES    = 0x81000300 + 0x22,
MSGR_E_REMOTE_LOGIN    = 0x81000300 + 0x23,
MSGR_E_INVALID_FRIENDLY_NAME    = 0x81000300 + 0x24,
MSGR_E_SESSION_FULL    = 0x81000300 + 0x25,
MSGR_E_NOT_ALLOWING_NEW_USERS    = 0x81000300 + 0x26,
MSGR_E_INVALID_DOMAIN    = 0x81000300 + 0x27,
MSGR_E_TCP_ERROR    = 0x81000300 + 0x28,
MSGR_E_SESSION_TIMEOUT    = 0x81000300 + 0x29,
MSGR_E_MULTIPOINT_SESSION_BEGIN_TIMEOUT    = 0x81000300 + 0x2a,
MSGR_E_MULTIPOINT_SESSION_END_TIMEOUT    = 0x81000300 + 0x2b,
MSGR_E_REVERSE_LIST_FULL    = 0x81000300 + 0x2c,
MSGR_E_SERVER_ERROR    = 0x81000300 + 0x2d,
MSGR_E_SYSTEM_CONFIG    = 0x81000300 + 0x2e,
MSGR_E_NO_DIRECTORY    = 0x81000300 + 0x2f,
MSGR_E_RETRY_SET    = 0x81000300 + 0x30,
MSGR_E_CHILD_WITHOUT_CONSENT    = 0x81000300 + 0x31,
MSGR_E_USER_CANCELLED    = 0x81000300 + 0x32,
MSGR_E_CANCEL_BEFORE_CONNECT    = 0x81000300 + 0x33,
MSGR_E_VOICE_IM_TIMEOUT    = 0x81000300 + 0x34,
MSGR_E_NOT_ACCEPTING_PAGES    = 0x81000300 + 0x35,
MSGR_E_EMAIL_PASSPORT_NOT_VALIDATED    = 0x81000300  + 0x36,
MSGR_E_AUDIO_UI_ACTIVE    = 0x81000300 + 0x37,
MSGR_E_NO_HARDWARE    = 0x81000300 + 0x38,
MSGR_E_PAGING_UNAVAILABLE    = 0x81000300 + 0x39,
MSGR_E_PHONE_INVALID_NUMBER    = 0x81000300 + 0x3a,
MSGR_E_PHONE_NO_FUNDS    = 0x81000300 + 0x3b,
MSGR_E_VOICE_NO_ANSWER    = 0x81000300 + 0x3c,
MSGR_E_VOICE_WAVEIN_DEVICE    = 0x81000300 + 0x3d,
MSGR_E_FT_TIMEOUT    = 0x81000300 + 0x3e,
MSGR_E_MESSAGE_TOO_LONG    = 0x81000300 + 0x3f,
MSGR_E_VOICE_FIREWALL    = 0x81000300 + 0x40,
MSGR_E_VOICE_NETCONN    = 0x81000300 + 0x41,
MSGR_E_PHONE_CIRCUITS_BUSY    = 0x81000300 + 0x42,
MSGR_E_SERVER_PROTOCOL    = 0x81000300 + 0x43,
MSGR_E_UNAVAILABLE_VIA_HTTP    = 0x81000300 + 0x44,
MSGR_E_PHONE_INVALID_PIN    = 0x81000300 + 0x45,
MSGR_E_PHONE_PINPROCEED_TIMEOUT    = 0x81000300 + 0x46,
MSGR_E_SERVER_SHUTDOWN    = 0x81000300 + 0x47,
MSGR_E_CLIENT_DISALLOWED    = 0x81000300 + 0x48
MSGR_E_PHONE_CALL_NOT_COMPLETE    = 0x81000300 + 0x49,
MSGR_E_GROUPS_NOT_ENABLED    = 0x81000300 + 0x4a,
MSGR_E_GROUP_ALREADY_EXISTS    = 0x81000300 + 0x4b,
MSGR_E_TOO_MANY_GROUPS    = 0x81000300 + 0x4c,
MSGR_E_GROUP_DOES_NOT_EXIST    = 0x81000300 + 0x4d,
MSGR_E_USER_NOT_GROUP_MEMBER    = 0x81000300 + 0x4e,
MSGR_E_GROUP_NAME_TOO_LONG    = 0x81000300 + 0x4f,
MSGR_E_GROUP_NOT_EMPTY    = 0x81000300 + 0x50,
MSGR_E_BAD_GROUP_NAME    = 0x81000300 + 0x51,
MSGR_E_PHONESERVICE_UNAVAILABLE    = 0x81000300 + 0x52,
MSGR_E_CANNOT_RENAME    = 0x81000300 + 0x53,
SGR_E_CANNOT_DELETE    = 0x81000300 + 0x54,
MSGR_E_INVALID_SERVICE    = 0x81000300 + 0x55,
MSGR_E_POLICY_RESTRICTED    = 0x81000300 + 0x56

Edit: Wow, that's a lot...

Try this:


--------------------------------------------------------------------------------

MSNTS: Error 0x81000303: Sorry, we could not sign you in because the sign-in name you entered does not exist or your password is incorrect.(821037)

--------------------------------------------------------------------------------

The information in this article applies to:
MSN Messenger
MSN Premium
MSN 9 Dial-up
MSN Plus
MSN version 8.5
MSN version 8.0

--------------------------------------------------------------------------------

SYMPTOMS
  When you try to sign in to MSN Messenger, you receive one of the following error messages:
Sorry, we could not sign you in because the sign-in name you entered does not exist or your password is incorrect. If you have forgotten your password, please click Help in the main Messenger window, and then click Help Topics for instructions on resetting your password. 0x81000303
  MSN subscription customers only: The Details section of the MSN Status and Support page may display the following error message:
81000303: MSN Messenger encountered an unknown error.
MSN subscription customers only: For additional information about how to use the Status and Support page, click the following article number to view the article in the Microsoft Knowledge Base:

818713 How to Use Status and Support


CAUSE
  The cached Microsoft .NET Passport credentials may be incorrect.

  Your .NET Passport account may have expired.

  Your password may be incorrect.

  Your .NET Passport account may be blocked because of the number of retries.

  You may be experiencing network latency (slowness) when you sign in to MSN Messenger.


Probing Questions
  "How long ago did you last use this account? "

  'Can you sign in to the .NET Passport member services e-mail name as sign-in name (EASI) account or to your Hotmail or MSN account? "

  "Have you set MSN Messenger to automatically sign in?"


THINGS TO TRY


  1. Agent: Look for server or network outages. For additional information about how to look for server or network outages, click the following article number to view the article in the Microsoft Knowledge Base:

326097 MSN: How to look for server or network outages

  2. Clear the Internet cache and cookies For additional information about how to clear the Internet cache and cookies, click the following article number to view the article in the Microsoft Knowledge Base:

326320 MSN: How to optimize Internet Explorer settings

  3. Visit http://login.passport.net, and try to sign in with your e-mail account. If you receive the following error message, your password may be incorrect or you may have an expired account:
“Type your .NET Passport sign-in e-mail address and password correctly. Remember, passwords are case-sensitive.”

Note If you have not used your Hotmail or your MSN e-mail address for a long time, your account may be deactivated. To register for a new Hotmail account, visit http://hotmail.com/cgi-bin/register

  4. If you are using a non-MSN sponsored domain account or an EASI account and you have not used it for more than 365 days, the account may be unavailable. If this problem occurs, visit http://passport.net and register for a new .NET Passport. 
  5. To reset your.NET Passport credentials, visit http://memberservices.passport.net Click Help, and then follow the steps to change your password, to reset your password, or to request your password. 
  6. Your .NET Passport account may be blocked after several retries. Try again later, or contact MSN support for help with unblocking your account. 
  7. If you can sign in to your Hotmail or MSN account or you can access your EASI account profile through member services, use the following method:   To sign in to MSN Messenger, follow these steps:   a. Start MSN Messenger. 
  b. On the File menu, click Sign In. 
  c. Enter your e-mail address and password, and then click Sign In.


  If you still receive the error message, you may be experiencing network latency or there may be a problem with the .NET Messenger service.


  8. MSN Subscription customers only: Generate a Status and Support report file, and then e-mail the file to MSN support. For additional information about how to use the Status and Support reporting functions, click the following article numbers to view the articles in the Microsoft Knowledge Base:

818713 MSN: How to use Status and Support




818695 How to send and view a Status and Support file

  9. If the error prevents you from using the Status and Support page:   a. Record the error message and the circumstances when the error occurred, and contact one of the following support groups for help with this problem   Contact MSN support, http://support.msn.com

  Call MSN Technical Support: (800) 877-9580



  10. Agent: Contact an L2 agent for additional help or approval for an escalation. For additional information about how to contact an L2 agent, click the following article number to view the article in the Microsoft Knowledge Base:

326434 How to escalate to L2

yeah i can log into my hotmail account..and i can log onto my messenger from other computers but mine..??