Hi, charliegirl - what makes you think someone is tampering with your account?
Regarding the event log viewer, I don't think that is any indication of anyone accessing your account.
Firstly, I would guess that when it says you have been logged out it means you have simply lost your connection to the server. I these cases Messenger automatically logs in again after about a minute. If there is less than 2 minutes between the time you were signed out and the time you were signed in again, I'm pretty sure that would be the explanation for this.
Secondly, If someone were to sign in using your account you wouldn't be signed back in, and the last entry in your Event Log Viewer would be 'You signed out'. The event log can only detect events coming from your computer.
The only exception that comes to mind is if someone were actually using your computer, as in someone in your house. Have you been mean to your brother lately..?