I've had a problem for a while with the 64bit Ultimate edition. It would lock up and you could move the mouse pointer, but nothing else worked and a full reset was the only way out.
I tried all of the fixes that have been suggested on various forums, including disabling reporting back to Microsoft developers, but it persisted.
I seem to have finally tamed it by disabling screen blanking. I've left the screen saver running, but no other shutdowns of anything and it's now stabilised. It's no big deal, as I have a KVM switch and I simply switch to another PC which can blank its screen.
The mobo is an Asus P5QC running a Quad Core processor with 8Gb ram and an ATI graphics card with a fully Windows 7 driver downloaded from the AMD site.