Not clear what could have caused this, but it is likely that you will need to uninstall MSN Messenger and reinstall it to get over this problem. The solution to your problem is going to be working out why you can't uninstall the copy you have.
Try (if you haven't) uninstalling it via control panel. Try microsoft.com & google.com to see if you can find a seperate uninstaller / clean up utility.
Once you have removed the old copy, you might find a new installation works.
Ant, February 2005