I have an older computer i got from someone, I tried to boot, got a hard drive error, so i got a new hard drive, and installed windows, all went well, then while downloading avg, it shutdown, then restarted couple min, later. Here is what i have tried, different power supply, changed out cd, checked ram with mem test. but it just keeps shutting down, usually within 5 min, then restarts after a couple min. Even does it when i boot it, and just leave it alone.

If you came into my shop, and relayed the identical information, one of the first things I would suspect is overheating. I would remove the CPU FAN Heatsink, clean them, remove and reapply thermal paste and reinstall. I would check all vents and fans to insure they were not plugged with dust. If that didn;t do the trick, I would boot into safe mode and try to run a malware scan. On a side note, I'd skip AVG and use online scanners like Trend Micro or Bit Defender which don't sit in your system tray and hog resouces because they are not running on your computer, then add an extra layer of protection for spyware with SuperAntiSpyware or Malwarebytes Antimalware. Craig Minnesota Remote PC Repair