I am having a similar problem now with one of my rigs. Mine will turn on, load into windows (takes 5 minutes though), and then reboot right after you login. It didn't really bother me as I never shut it down so didn't have to deal with it, but now it is rebooting randomly. I swapped PSU also and it kept happening. Turns out the HDD the OS is installed on is going bad. SMART eventually told me failure was immenent(sp?).
I'd say maybe you HDD itself is bad... but then again that shouldn't stop video from working...