My PC, running Windows XP SP2, has begun restarting itself during the normal startup sequence. It will do this two or three times before becoming stable. Besides being a nuisance, I am concerned that one day it may start doing this in an indefinite loop, so that I don't get a chance to use it. This hasn't happened yet, but I wonder if it might. I suspect it could be something I have installed. I have checked for viruses, so that's not it. I have made restore points, but am reluctant to backtrack if I can avoid it. Has anyone else had this problem and solved it, or alternatively is there a way of detecting what might be causing it?

I would guess a PSU or a memory problem. I would download Memtest or equivalent (Free) memory testing program and run it. I would suggest backing up all data ASAP.