A full story and fine details of the machine may help. Some of those older Athlons are dieing due to neglect (cpu heatsink falls off) or other issues.

The wild guess could be motherboard, cpu, video card, hard disk, CD/DVD failure too. You don't know till you strip it down. Here's some troubleshooting guides.

http://www.google.com/search?hl=en&q=TROUBLESHOOT+A+DEAD+PC&btnG=Google+Search

Bob