I upgraded to Windows 7 and have had constant problems with random hangs ever since.
After weeks of stress I took the side panel off to see if I could smell the problem (ok, I'd tried every rational solution). Not had a hiccup since. So - the solution that worked for me is to take the side panel off the box. Put the panel on and it stops within a minute or two.
Someone told me to upgrade my bios - that seemed a little extreme (plus I couldn't find my model MB bios on the MSI website).