Tried everything but buying a new system over the last year!! You guys have been great and very patient. I am at a total loss at where to go from here.
Compaq 5320US 1.5 with 768 mb memory and 256 mb Nvidia card.
Not heat, not mem, not power.....no idea? Would restoring it with system software (from compaq) help?

How do you know it is not heat, memory or power?

Besides those things, systems lock ups can be caused by a failing video card or a failing motherboard.

Hey Digerati...thanks for siging back!
Those things have been exhaustedly looked at over the year mainly by jephree. Its a Compaq 5320US. Video card fairly new and system was working fine until either of the following occurred: I played an online baseball game, or updated to SP2. Cant quite remember which I did first but thats all I can attribute to the start of the end.

Any advise is much appreciated. Do CPU go bad slowly or just die? Taskmanager often shows usage at 100% when just staring at screen, fan ramps up to max, time only thing to return to usable state. Obviously sysytem not responsive during that time. Typically associated with gaming or video (espn arcade, etc.) Very choppy video, the freeze for acouple seconds, then picks up at new phase of game or video. Audio choppy, during this stage as well.

It is not likely SP2 caused it, although SP2 did require a few hardware driver upgrades.

100% CPU usage is often a sign of malware infestation, which can cause a system to bog down and freeze. You might want to run through the sticky, Cleaning Out Malware.

Choppy video is also a sure sign of video card stress - either by heat, or by inadequate PSU support. Could be card drivers too.

CPUs generally work, then die suddenly - typical for digital devices. But heat can cause them to lock up too. You might want to blast a desk fan into the open side of the case and see if it happens.

Inspect the motherboard for leaky capacitors. Leaky capacitors are a common cause of sudden system lock ups and reboots. Look for white to dark brown dried foam leaking from the tops or bottoms of the capacitors. The capacitors look like tall soda cans, many of which surround the CPU socket. Bulging capacitors are a sign leakage is about to occur. A motherboard with leaky capacitors can be repaired, but generally it is more cost effective to replace the motherboard.