AdWatch is a part of Adaware SE, see if you got that running (there should be an icon in the systray....besides the clock)...if so close it. Then try install MSN messenger again. I that doesnt work follow the steps i described in my previous post
I really can't explain more clearly, give it a try..if you truly can't work it out pm me and i'll see what i can do
It most likely is AdWatch..if the above does not work you could try doing the following aswell:
1) Click Start > Run
2) Type: msiexec /unreg <enter> and wait until it appears to have finished.
3) Go back to Start > Run
4) Type: msiexec /regserver <enter>.
5) Try to install MSN Messenger 6.2 again
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