1) Make sure the other program is MSN Messenger not Windows Messenger...
2) Go to the Control Panel (classic view) and look for add/remove programs...how many versions of MSN Messenger are there? How many versions of MSN Messenger are there?
3) Do you use a patched version of MSN Messenger? (Games Patch, Polygamy, Skinned ect.)
4) Update to the latest version of MSN Messenger (6.2.0137) from
http://messenger.msn.com/