Cause Dude it's a Dell.

I just saw you post today. Ihave a maximum pc mag that talks about BSODs. It says when you get the IRQL on the screen it will tell you the offending driver or file. It says that it usually happens when U go from ati to nvidia or vice versa and remnants of the old driver is left.Check for conlicts in device manager.It says also to use a program called "driver cleaner"(www.drivercleaner.net). It will clean up the remnants of you display driver and the reinstall an updated driver.A couple of other things it says but I doubt is that you system is overclocked and the bus speed for the VC is O/C and causing the error and lastly it says some virus scanners and backup utilities can give you this error(dont run more than one V scan). Hope this helps
Sarge