I started experiencing this problem on my network when we upgraded our clients from XP to Win7. Turns out in my case it was due to our FOG server. We had it configured to auto restart/log off people after 3 hours of inactivity. Unfortunately FOG does not seem to correctly detect inactivity on Windows 7 and so it would restart our client PC's exactly 3 hours after they had logged into the machine.
I disabled FOG's Auto Log off feature and I decided to chose a screensaver based auto log off program called Screensaver Operations from GrimAdmin. It works very well for us and I am happy it is available.
