|Is it Blue Screening or just shutting down?|
I did a Google search, there seems to be info out there - i.e.- RealTek sound issue, Win7 release candidate issue (trial OS expired- therefore shuts down periodically), etc etc.
Scheduled shutdown task set somewhere?
Any bad capacitors on the MOBO?
I'd check for Malware too - run Emsisoft (formerly known as A2), MalwareBytes, SpyBot S&D, and several anti-rootkit scanners - GMER, Radix, RootKit Revealer etc.
Just another stupid saying...