Mr B, i have not seen this before, but i have seen windows do spooky stuff, so while i can not tell you what's wrong exactly and where you can follow up on things, i can recommend a "its just gonna be fixed" technique... hopefully anyways.
granted that system restore does not work... (and thats the first thing you wanna try

)
figure out what OS version he is running, and find his key to begin
dig through your pile of cds (and i know that any self-righteous computer person has one) and find the installation cd for that version (or newer), worst case, you may have to download it.
put the media in the drive, and reboot to start installation.
in windows setup screen, you are going to say continue, agree to eula, and get to the part that searches for previous windows installs
it will find one, what you want to do is either update or repiar the windows installation (notice the first screen will ask you if you want to repair... skip that one, you gotta get to the part that finds previous installations)
then it will copy all the system files over the current ones, refresh the system registry entries, in 15-20 minutes, it should be fixed
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And remember that great question that
Pierre-Simon Laplace and
Sir Isaac Newton,
Andrei Markov and
David Hilbert,
Richard Feynman and
Enrico Fermi,
Albert Einstein and
Edmund Halley did not come to ask throughout all of their dedication and work: "Who the hell is IMing me?"
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