One of my PCs started messing up a while ago. It would get to the windows boot screen(the one with the progress bar), make a weird sound, and restart. I was wondering if it had to do with windows since the same happened to this and reinstalling windows fixed it, but I dunno if thats the same for the other one.
It just wont boot at all? Or it takes a few times? If You can get it to boot. Look in the event viewer for the error. If its making a sound like a beep.. its the BIOS.
put in the xp disc and run the repair wizard. If that doesnt work, try running memtest, download it and burn it to a cd and boot into it when you start up your computer, and run a few tests on it. If no errors turn up your ram is fine, if there are errors your memory might be kaput. If those fail to fix or locate the issue a windows reinstall may be in order.