Maybe it might help if you tell us what file it is missing..
I think there are three possible causes:
1) Bug in beta messed thing up.
2) You have a virus and/or spyware/adware on your computer.
3) Security software or other program is blocking the installation.
So what you can do is try to use System Recovery (start->programs->accessories->system tools) to go back to a date before you installed MSN Messenger 7 beta. If you don't have a virus scanner you can get AVG for free
here or use
housecall which is a online virus scanner. Cleaners for spyware and adware can be found
here (Ad-Aware) and
here (Spybot S&D). And finally try to close as many programs as you can because something (most likely security related software) might be blocking the installation.
Things like this are why non experienced people should not install beta products which might cause harm to your system.
