I have been researching this BAD_POOL_CALLER error code and everyone suggests certain downloads. But, I cannot do a download if I cannot restart PC.
Machine is a Dell Dimension 521!, BIOS V 1.1.10. Installed memory 1.0 GB 667 MHz Windows XP OS. This machine is at our 2nd home and we returned after 3 months. Both my husband and I were logged on and I had just hit the button to start update downloads. Within 30 seconds power went off and computer shut down. When I restarted, the dreaded blue screen and nothing I try will let me restart. Tried safe mode. It gets as far as "Loading Windows XP", then back to blue screen. I have service pack 3 installed. Help!!

I suspect your computer was damaged by the power shut down... and may be difficult to repair. Get an estimate from Dell or a qualified, licensed technician, then talk to your Insurance company... If may be fixable or it may not, but I would replace the board under an Insurance claim if there is any question about its long term reliability.