Two days ago my son's 6-month-old computer was working fine. Yesterday it started rebooting just before getting to the desktop (could enter BIOS and system would stay up though) and now I can't get it to recognize the keyboard while still in system boot. Any ideas about what is going on?

Could be a wide variety of things. First of all, is the keyboard USB or DIN/PS2? If it is USB, Get another keyboard with the PS2 style connector and try it. As far as the rebooting goes. You probably have a corrupt file in windows. What the screen closely while it is booting, just before it reboots, a blue error screen is probably flashing. These can be a pain to read, but it is probably there. You can also try booting into safemode. You can enter safemode by pressing F8 just after the BIOS post and before the XP loading splash screen appears.