I'm having the same issue; windows automatically logs me off from the welcome screen. Aside from a variety of other attempted fixes (most of them stuff like system restore and the like) I tried the suggestions from ScinTex and koisidis. Neither worked.
In each case, when I typed the commands they both recommended, the system told me the appropriate file had been copied, but when I rebooted and checked, it didn't change anything. Anyone got any other suggestions before I just give up and get a new HD?