Explorer.exe dissapears.?

Yesterday my brother used my old pc in my room and today when i started up the pc (win xp sp2) my taskbar and icons were gone. When I looked at the running processes I noticed explorer.exe was gone. I started it manualy but it disappears after 2 seconds or so. I have avg anti-virus, spybot, spyware terminator, ad-aware. They did not find anything. I searched on google and found something about vundo. I installed a remover and it helped, the first time. When i rebooted however –> same problem. I can still use my computer but have to do it through ctrl+alt+del, thats no way to work. My brother downloaded a keygen for some game, I deleted it but the harm was done. Anyone knows a solution that doesn’t include reinstalling the OS?
system restore disabled, lost win cd, have one from my other pc, could I use that one?

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Note : backup new works

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