New PSU, shuts off

AcerPower SK50, replaced a failed PSU (SHAW EVO 775Max) with a new PSU (SHAW EVO 975Max). Booted into Windows 7 fine. Shut down, removed the CPU heatsink/fan to removed the dust, replaced into unit. Kept bluescreening on 'C000021A fatal system error'. Went through the usual troubleshooting;

Reseated Graphics card
Reseated Memory
Reseated cables in all devices
Tried Last Known Good Configuration/Safe Mode/etc
Booting with Hiren's Boot CD to Mini Windows XP, just shut down before getting into the GUI
Tried to do a repair install with Windows 7 USB, just shuts down when "Loading files" appears
Clear CMOS

Strange.

Did you reapply the thermal paste after you removed the heatsink?

No I didn't Steve.

A cpu with no/poor thermal paste will shut down in under 20 seconds...