Windows 7 is shutting down unexpectedly, according to the logs
Jeff actually blogged about this.
It's for Windows XP but I suppose this still works for Windows 7. It basically comes down to flipping a switch in the Local Group Policy Editor (gpedit.msc).
If you are using the Home versions of Windows, you won't have the Group Policy Editor, but you can disable it using a registry hack.
And if you don't want to manually hack the registry, I've got a downloadable version for you:
Prevent Windows Update from Forcibly Rebooting Your Computer