windows xp logs on then logs off

When I start my computer, I make it to the "choose user" page of XP, but when I click on a user, it says it's logging on and then it logs off immediately after. I tried to boot in safe mode but the same thing happens.  How can I solve this problem? The problem is, I don't have the system disks, so I can't use them to reinstall Windows. Does anyone know if I have to reinstall Windows? And if I do, how do I get the system disks?