I've got an old machine, a 1.8 GHz Pentium 4. It's been very cranky lately, acting like it's got an overheating problem. I've upgraded from 512M to 2G of RAM. Yesterday, I got a 2.6 GHz processor (used) and put that in, cleaning out the fan cooler and replacing the thermal grease. The machine booted right up, but froze halfway into a game of solitaire after starting Windows XP successfully. Subsequent re-boots were unsuccessful. I configured the BIOS to run the full memory test, which takes a while. After it gets all the way through the memory, it just stops.During the memory test, if I press F2 to go to the BIOS Setup program, the memory test stops and the machine once again hangs.If this follows true to form, when I go home this afternoon and power up, it will run fine, initialize Windows, run for a while and hang again. This is basically the same performance I've seen with the old processor and memory, so I don't think the upgrades have had any effect on the problem.Has anyone else seen a problem like this? How did you fix it?

What mobo do you have and what BIOS version? Exactly what model is the new CPU?When you install a new CPU you need to first check if you need to update BIOS. An older BIOS version might not support all CPUs. Did you check this prior to installing the new CPU? Chipset drivers might need an update as well. The manufacturer of your mobo will probably have support and support forums available.The new CPU or your mobo might be faulty. I would try to reinstall the old CPU and make sure that the correct BIOS version is installed and that the system is stable before proceeding to install the new CPU again.