From what I can think of, you have two options: You can either uninstall MSN Messenger and reinstall it, or you may be able to use System Restore.
Let me know if this helps.
I use Windows XP. I had MSM Messenger and AIM installed on my computer. I wanted to delete AIM and did so via the Add/Remove programs option. Unfortunately when I was asked whether I wanted to delete a shared file, I said Yes instead of No. I didn't make a note of the deleted file. I now can't connect to MSM messenger and presume that the deleted file was one that was also required for MSM messenger. Does anyone know which file I deleted and can you tell me a way I can restore the file without having to reinstall Windows XP? (I tried to use the Registry Edtior but that didn't work as I didn't have the option to revert to the "Last Known Good Configuration".

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic