Desktop

Replace the power supply.

While it may be the power supply itself, I suspect that the CPU is over-heating.

Start by running a hardware monitor program such as Mother Board Monitor:

http://mbm.livewiredev.com/

Then you can see if the CPU is getting hot, or if there is some other hardware issue.

Just because a fan is spinning does not mean that it is running fast enough to cool down the CPU.

Chas

Chas has a point. You may also find the processor's fan is noisier than new. You can remove it and clean any dust balls out of it ir really dirty or simply **** it out.

Most fans have a small sticker covering the bearing in the center of the fan. remove the sticker add ONE drop os sewing machine oil and replace the sticker to keep the bearing sealed. This will help keep the fan running at the correct speed.

OR

Just go but a new fan for $8. :-)

Most newer M/B have system monitoring within the BIOS. Find the monitoring screen there and you'll see th CPU & chassis temperature, voltages, fan speed etc. You can leave it there to see if anything changes significantly. If you CPU heatsink is plugged with dust. You'll need to **** it out (compressed air arosol cans are available)P.S. can also build up large quantities of dust so **** the all out. Most likely an over temp problem and lastly the power supply may fail or over heat with time. If windows based monitoring software is available for you M/B install it and you can run and monitor for the OS. Alerts will appear at the alert thresholds set. BIOS also has a preset reboot or shutdown when CPU reaches a preset temp, again CPU cooling problem (plugged heatsink and/or fan on CPU faulty)replace as sensors indicate problem area.

Check your power supply's fan. Or clean it also like cleaning the CPU fan. If you can, examine the power supply if there is a sign of slightly burned capacitor. Clean the dust from it so you can see the tinier parts. Have the capacitor replaced. If not, do not use it. You may regret it later on. I mean the damage that it will do to your motherboard, then to your CPU, and so on and so forth.

The fans might be moving, but are they moving fast enough.

Dust on the blades, bad bearings, low voltage. Use canned air and **** the blades off. You might want to take it out on the porch first. You will be shocked at what will be in there.

Lee

Check your cmos and see if it has the system health monitor section.

You will be able to see temperature and usually fan speed on the processor.

The supply is a different story.