Computer shuts off when I run video conversion programs

I am having an ongoing problem with my PC suddenly shutting down for no apparent reason. It usually happens when I am running a video conversion program (e.g., avi to mp4; DVD Shrink). At first I thought it was one of my hard drives overheating, so I had the operating system moved to the IDE drive from one of the SATA drives and upgraded the fans. However, the problem is continuing. Disk Sentinel tells me the disks are all fine, and I replaced my video card just a few months ago. I am running Windows XP Pro on an AMD Dual Core 4200+ with 2 megs of RAM. I have three internal disks and one external disk.
Thanks!!
:confused

 

These things can be pretty hard to diagnose, the first thing I would do is to look in Administrative Tools > Event Viewer, just after it occurs (to make it easier to identify) to see if I could get an Idea what was involved in the crash. Often crashes don't give the system time to ost an event so its an iffy prospect at best. How about your cpu heatsink? whens the last time you removed the collection of hair and dust from the fins, or your cases filter at the front if it has one could be clogged as well.

 

This was my guess. Video conversion is a very CPU intensive activity, and can easily cause a thermal problem on an improperly cooled processor. In addition to cleaning the case fans, and the CPU fan and heatsink, it may be a real good idea to also remove it, clean the CPU and the base of the heatsink (clean 'em both REALLY well and REALLY carefully) and apply a fresh layer of thermal snot. Don't over-do it. While your at it, check the fan to make sure it's spinning smoothly and easily. Give it a 'flick' with your finger; it should spin easily and continue spinning for a while. When it stops, it should have a little bit of "bounce back". This means it should kind of spin 'backward' for a tiny bit right when it stops.

 

I think I've come to the conclusion that it's probably the CPU overheating. One thing I didn't mention: I installed a new CPU (upgraded from a 3400+ to a 4200) but I didn't upgrade the CPU fan. Although did a lot of research before I installed it, I wonder if I (a) did it right, or (b) should have upgraded the sink fan. I think I will take it in & have them chack the installation & replace the sink fan.
Sound like a good idea??