Computer reboots even in the bios setup pages

[andread]

(PIII 500Mhz, 128 to 512 M, Matrox G400)
Hi, two days ago my computer started to reboot (without warning, just a black screen) every 5-10 minutes.
I started to remove various cards (NIC), extra HD, and replacing memory chips, but nothing changed.
Checked the CPU and power fans too, and pushed to the max the air conditioning system in the house too!!!
I thought it was caused by some software in Windows 2000, but even if I just enter in the bios setup page, after 5 minutes reboots!!!

Please give me some advice, I'm going crazy, thanks.

Andrea

[LooseChippings]

I would seriously consider doing a full system virus scan using AV software with up to date definitions.

[LooseChippings]

Try disconnecting your HDD and Optical Drives and see if the system reboots.

It could be bad memory or CPU overheating.