Bootcamp Win7 - won't boot from Windows partition or even from the Win7 install DVD

Late 2011 Mac Mini / Lion / Win 7 64-bit Home Premium.

Incumbent Windows partition has happily run for heavy gaming (World of Warcraft) for some eight months. Now it stops on boot-up before the Windows logo is displayed, with a very dark blue blank screen. Attempts to boot from the Windows Install DVD yield the same result, portending that there is more than a corrupt MBR at work here. I have reset PRAM and SMC, just for good measure. There have been no recent changes to hardware / peripherals.

Rebuilding the Windows partition with Bootcamp stops at same Dark Blue Screen of Death. Will start Windows sometimes from first switch-on / dead cold Mini, but then not again in the same day. OS X Lion mode runs perfectly, booting up as usual with no quirks or delays.

Hardware? Seems unlikely, except for that suspicion of starting from the dead-cold start.