Computer Reboots Continuously - Even during a new install of Win XP

[help_regina]

Hello,

I have a computer (Acer, now 80G hd, 256MB mem and Pentium 4) that continuously reboots, even during a clean install. I installed two anitvirus s/w packages (Anti-Trend V S & Norton Antivirus) and Spy Sweeper. The computer works for so long and then reboots. Tried to recover the old set-up, w/o success. I then formatted the hd and tried to reinstall the op system, Windows XP. I'm unable to install the op system without it rebooting in the middle of the install. Then Installed new memory module. Same problem. I can't complete an op system install. My last resort was to purchase a new hard drive (80 GB) to eliminate any possibility of a dormant virus. Installed the new hard drive, began the op system install, when it was nearly finished, the computer rebooted. If its not memory, a virus or hard drive, and the cables are tight, what is making this computer reboot? Please help...I don't have any idea what might be causing the problem. Even tried to install Windows 2000 on the new HD. Same problem. Please Help!!!