First things first, make sure you aren't on the internet on the infected computer. Secondly, it isn't your MSN that is giving you viruses. What AV do you have installed on the computer?
On the infected computer (without internet connected) go to Task Manager and look for suspicious processes running. Ones like services.exe. It is a System program, but if it was a "hidden" virus it would say the computer's name instead.
Since your AV didn't catch it, you may need to install a separate one from an installation disc, flash drive, or an external HD, and try to scan it with that. Avast! is good, and i've heard Kaspersky is also good.
Hopefully the info helped. Also, wrong section =/.