if your bro hanst done many things then all the viruses and trojans and god what else is the cause for sure, make sure you boot in safe mode like swordfish said, then do couple of scans, 1 for viruses if u already have a AV software then its good if not search AO for 'Free AVs' and ull find a ton of them, also download ad-aware and sbybot S&D, they will clear off everything i think, and make sure to update them or it'l be useless...after all that i'd uninstall msn and reinstall with newwest version.
tell us later how it went