A puzzling fault, given what you've revealed so far ....
I would be thinking along the lines of a problem in the OS files or registry area. Is it XP or Vista? If it's XP, does it boot OK in safe mode?
I have recently had a few XP machines displaying similar behaviour in recent months. It's been "cured" (if one ever dares to claim that) by running a registry cleaner program, and/or a system re-install/repair using the original supplied XP disk. Caution is required if you were to go own this road. It's relatively easy to select the wrong option.
The solving of the problem by removing the power source and then re-connecting is a strange one. Given that the 420 appears to be desktop, it may be significant, or it could be co-incidence. I've seen a few power supplies fail in recent months also.
If it's still coming to life at times, I'd be backing up my more valuable data as a top priority.
Perhaps you have more clues or a fix by now ...?
regards,
FoR