I've left Windows 7 (6.1.7600) running during the night several times and found that the machine will often experience a BSOD (it doesn't explain in more detail why) or I get a message saying SuperFetch has stopped working.
runs by default, and it works by monitoring the programs you use regularly and giving them priority over Windows background tasks - it pre loads the programs into memory. When you leave your computer is idling the background tasks will run, and when they've finished Windows attempts to reload the program data exactly as it was when you stopped using it.
The ideas is that when you come back to your machine the programs will be up and running faster and more efficiently.
If Superfetch is giving trouble you can disable it. You might want to give it a try - you can always enable it again if you find it affects the way your machine runs.
1. Open the Start Menu.
2. In the search area, type services and press Enter.
3. Click on Continue in the UAC prompt.
4. Scroll down and right click on Superfetch and click on Properties.
5. Click on the 'Stop' button and on 'Disabled' in the Startup type: drop down menu.