BSOD at random times: Driver IRQL Not Less Or Equal

Hello, I built this desktop approximately 2 years ago, but only just recently started experiencing BSOD at seemingly random times. The system hangs for a couple seconds, then goes blue and tells me "Driver IRQL Not Less Or Equal". I re-installed OEM Windows 7 Home Premium x64 about a week ago to try and clear up any problems, thinking it may have been a naughty program causing the BSOD, but...