Okay, the only thing I can think of is go to: 1. Start 2. Control Panel 3. Add or Remove Programs 4. Scroll down to Windows Messenger, or some variation of that. Then delete. Then try again.
If that does not work, I suggest using eBuddy. That is what I am currently using, and probably will for my life, since my computer had that virus a while back. The cool thing about eBuddy is you can put all of your messenger accounts (msn, yahoo, aim, myspace, etc.) into one account and talk to EVERYONE from those ims at one time in one window without downloading anything. I put my myspace, yahoo, and msn together in one account. And, because it is not part of your computer, it is less likely to contract viruses.