PC Reboots on startup

I have a PC with windows me, it has recently started to reboot whenever windows starts, I ran a few checks and I thought it was the hard drive, so I have replaced it with a Maxtor 80gb, the bios seemed to pick this up ok, I tried to reinstall windows me and I still receive the problem, I have even tried installing the following operating systems windows 98/2000/xp pro all of them reboot either as soon as windows starts or just before on the splash screen. My pc is currently running a 800mhz P4 proc, with 128md ram and a 32mb graphics card, it has a 200w power supply, as I said it has run with no problems for the last 5 years and apart from the hard drive there have been no other hardware changes, can anyone help?