I have found in general, that sfc /scannow will NOT fix corrupted files when run from within Windows; however, when I ran it booting to a command line from a Windows 7 DVD, it always worked. IF you're running it from inside Windows, try it from the DVD.
Also, suddent shutdowns like this, especially with the fan still running, imply an overheating problem. Does it drop after 5 minutes even after it's been off all night? OR does it only do that if you turn it back on after it's been running?
The other primary causes of shutdowns is a failing hard drive -- because Windows will attempt to write log files and other stuff as soon as it starts, and if those writes fail, the PC is liable to shut down.