My machine is as follows:-
1. window 7 Home Premium 32-32
2. AMD Seempron SI-42
3. 2GB DDR2 RAM, 2550GB HDD
4. 15.6" TruBite Widescreen Display
5. Matte Silver
This machine goes off anytime at all either on connected to the net, watching movies or busily preparing my report it will just switch off by itself.
Sometimes windows sneakily starts a 'windows must restart for updates' in the background without telling you. However I doubt that's actually the problem. Is the battery running dry? I think these are unlikely but it's good to make sure.