Many thanks to everyone for the kind words and help!
But.....
I did the complete uninstall and re-install but the same thing happens. It's about a 90 second load to the main menu then a 7min 30 sec load time to the mission (the quicker of the three loads I've tried). Then about a minute into the mission the computer just reboots with no blue screen, warning or otherwise.
The graphics card has 128MB of DDR over an above the 1Gb of RAM already installed.
I never had a problem playing SH3 (no mods, but patched to the latest) - its only since the introduction of GWX2
Thanks again for the help
Oh... and what or how do I "checksum"?