It could be MSN servers having trouble (if it was working fine before and nothing was changed in the firewall/network settings), but if it lasts long you can try the following:
Go to ....
start > run > type: regsvr32 softpub.dll <enter>
start > run > type: regsvr32 initpki.dll <enter>
start > run > type: regsvr32 mssip32.dll <enter>
If you still can't login after that, look at the first two posts in:
Possible solutions for "We were unable to sign you in to .NET Messenger"
If you can't reach/view any microsoft site, the first command oughta fix that aswell (along with IE settings ofcourse, more about that in the thread i linked to)
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