WinXP rebooting problem

I’ve just installed WinXP yesterday. Everything was ok yesterday. This morning, WinXP started to reboot after some seconds, or after some minutes, it depends on the mood of the PC! ;-( In the same time, the power supply of one hard disk falls then comes back after the “auto-rebooting”. I have an Athlon 900 on Abit KA7 (VIA-KX133). I have 3 hard disks and a DVD on the 2nd slave. This hardware has always been fine. This problem appeared this morning, one day after installing winXP. Of course I checked the cables.
An idea?
Thanks.