Ok well I've had the same issue. My whole computer would shut down after being in game for roughtly 20 min. It also happened when playing Guild Wars 2. So don't fret it's not a SL thing nor is it any of the viewers.
I did extensive testing, and I can assure you it's not heat. My comp runs between 47 and 52C.
I then thought maybe it was dirty. cleaned the whole thing out top to bottom inside and out. Still didn't fix it.
Ran bios, registry cleaner, defraged, disk clean, and updated every single driver.....still no luck.
The answer........the power supply. I'm running a 750w PSU