WinXP hangs or reboots on startup

I have suddenly had a problem with my machine either hanging or rebooting on startup. It seems at the moment that I can get it to bootup eventually - after which it runs perfectly.The point at which it either hangs or the screen goes black and reboots is just after the screen comes up that asks me to select which account I want to boot into.Any ideas of how I can monitor the boot process to try to determine what is happening at that point?Thanks Barry