I figured it was a power supply gone wonky, so I swapped in a power supply from a dead computer (500W). System boots, I'm letting it run for a while to see if it stays alive. However, at boot up, it complains that the "card cage fan" is not alive; however, it lets me continue. Will the system processor shut down after too much heat automatically without damage to the processor? That's my main concern. I bought this comp in spring 2006. Seeing as it will be spring 2009, I think I'm still under dell's parts plan but I couldn't wait for the part since I need to use my computer.
You should try blowing out all of the DUST that has accumulated inside your computer. All that dust creates HEAT and can cause all sorts of re-boot issues. It has happened to me in the past, and was solved by simply blowing all the DUST out.
^^ So true, esp on laptops. I've had 3 laptops in the past year that were just shutting off and further inspection found a quick blast of all fans quickly solved the problems.
Yes, the computer will normally shut down when the CPU core temp reaches a certain threshold. I don't know how you replace parts in laptops, but on a desktop, the CPU case fan and heat sink are the parts you usually want to swap out. You also have to properly apply the thermal paste or you will have the same issues.
Its most likely the Processor. If it gets too hot to fast, it shuts down. Try what the others said by getting one of those spray cans that blow air. That will fix it 90% of the time. The other 10% of the time its either the power supply, or the special glue between the heat sink & the CPU (this was the case for me once because I had taken the processor out, then had to go get a computer specialist to give me some of that special glue that goes between the top of the processor and the heat sink. Also, dont forget to blow the dust off the motherboard too. Too much dust on that can be pretty bad for it.
A good clean always helps and then: Try running a full test at pcpitstop its free once you log in.. It will help point out where any problems lie... http://www.pcpitstop.com/pcpitstop/default.asp
Here is an explanation from another forum. http://www.fixya.com/support/t535634-dell_xps_400_card_cage_fan_failure
Most motherboards have one or more temperature sensors built-in. Install some software the monitors the temp continuously and displays the temp in your system tray. E.g: http://www.cpuid.com/hwmonitor.php