Silly question, but can you check your voltage, either through a mobo sensor software, or through the BIOS?
I had hard lockups once. Sound would repeat forever, last frame on the screen would stay there. No hotkey buttons would work, no CTRL ALT DEL, total hard lockup.
It turns out part of the problem was related to my PSU being a total "POS" -- overvolting my +12 and WOEfully undervolting my +3. Indirectly, I solved the problem by disabling the onboard NIC and using a PCI NIC (I guess that reduced the voltage requirements somehow, it was pretty messed up).
Anyways, when I got a good PSU later, the onboard NIC worked fine with no problems, no lockups, no nada no nothing.
Just thought I'd put it out there.