Actually Crispy I don't believe this is what he's looking for. That will completely disable the ability to Logoff.. In order to stop the system from logging off due to inactivity, follow these steps
1. Right Click on an empty area of your desktop
2. Select "Personalization"
3. Go down to the bottom right hand corner of the window that pops up and click "Screen Saver"
4. A "Screen Saver Settings" window should pop up. Uncheck the box next to "On resume, display logon screen"
5. Click "Apply" then "OK"
6. Enjoy your computer
PS; If this isn't what your looking for, please post back