Restarts Right After Booting

Well, my father told me this all started after he clicked yes on an error message that he had always pressed no on...he couldn't remember what the message said.I'm running Windows XP. So after pushing power, the computer passes POST, then goes to a screen saying Windows can not start normally and gives an option of going into Safe mode, trying to start normally, etc.If I select start normally, it goes to the black Windows Startup screen, showing a progress bar, then the screen goes black as if it's going to go to the blue welcome screen then just restarts, with the same loop over and over.In safe mode, it says what drivers it's loading, then quickly does a blue screen (too quick to read), then restarts.Do you guys think this is a software problem? Is there any way to fix it without reinstalling Windows?