I suppose this computer is yours, I mean, you do not use it for profit
Well, avast! could scan MSN Messenger with the IM provider.
If you disable avast! (right click the 'a' blue icon and choose the last option in the menu), could you login?
Do you use a firewall?
Are the following programs allowed to connect?
C:\Program Files\Alwil Software\Avast4\ashServ.exe (avast! antivirus service)
C:\Program Files\Alwil Software\Avast4\ashWebSv.exe (avast! Web Scanner) (allow server rights)
C:\Program Files\Alwil Software\Avast4\ashMaiSv.exe (avast! e-Mail Scanner Service) (allow email rights)
C:\Program Files\Alwil Software\Avast4\ashUpdSv.exe (avast! Update Service)
C:\Program Files\Alwil Software\Avast4\Setup\avast.setup (avast! Update executable)