Had you added new hardware to the problem PC? Such as an Ethernet PCI adapter card?
I had been chasing intermitent issues with my PC locking up which I thought was down to various recent activities. It turned out that my PSU (Power Supply Unit) was under powered for the few extra items I'm running, and had been for a few years!
Replacing my PSU removed quite a few issues which were niggles and I'd just lived with!
Your PSU's specs are on the PSU itself so you will need to open up your PC. Athlon and Pentium 4 CPUs run off the 12volt supply instead of the 5v. If yours is rated 15 amp or less, and you are running 2 HDDs for example, your CPU is the likely culprit.