My 27" iMac runs great with Win 7-32 bit, but if I leave it alone for a while, it will restart for no reason. About 20 minutes or so later, it will restart again, and will continue to restart every 20 minutes or so for as long as I leave the machine untouched.
As long as I'm working with it on the Windows partition, I have no problems. Also, it will sit without unplanned event for as long as I leave it if I'm running on the OSX partition.
I have changed settings for screen saver, go to sleep, and energy saver modes to "never."
It's just a nuisance as long as I remember to save my work and shut down all apps before I walk away from the machine. But I sure would like to be able to come back and find it just the way I left it.