I have an Inspiron 2600.  I recently removed the heatsink/fan assembly from the processor as I was trying to look at the processor to determine if I could upgrade from the current Celeron to a PIII or P4.  I put the heatsink/fan back on and booted up the computer.  Everything was for about 5 minutes, when the system just shutdown.  I turned it back on and then every 15 seconds or so, it would simply turn off.  If I let it sit for a few minutes, it would always come back on and boot into windows, but then would simply turn off, and subsequent tries to boot up would always result in the computer just shutting down.

 

This appeared to be an obvious over heating problem (especially given that I had removed the heatsink momentarily).  Every time I booted up the computer I would see the fan come on.  I finally removed the keyboard and the metal shield cover to expose the fan.  I turned the laptop on, and sure enough, the fan came on.  After about 5 seconds it just turned off, and never came back on!  I hadn't noticed this before because I would see the fan start to spin up and I just assumed it was continually spinning.

 

I let the laptop cool sufficiently, and then booted up off of the Dell Diagnostic CD.  I ran a fan test and the fan worked perfectly and passed.

 

So at this point, the fan has proven to start up (although it stops functioning after 5 seconds or so) and the Dell utilities report that the fan is fully functional.  I even watched the fan go through the test and it definately worked.  Why then will it shutdown each time I try and boot the computer?  Could I have broken a thermostat somewhere?  Is there a way to test the thermostat?  I could easily buy a replacement fan, but I am afraid that it will have the same problem, as my current fan is functional, it just doesn't come on when it is supposed to.  Any thoughts?