Computer Problem

Recently in the last month or so, my computer will just shut off during boot up. Most of the time before even windows starts to load. This only happens when starting it up after 8 hours. (I always shut my computer down before work so it is off roughly 8 hours a day).

No new hardware for quite some time. I can't see it being a software issue seeing as nothing has a chance to load before it shuts off. This power off only happens once, and then I am able to boot the computer normally. Any possible ideas what would be causing this issue?

I have been running this hardware configuration for at least a year I think. I'm not sure if my powersupply may be malfunctioning or what (like I said earlier, only happens once then runs normal).

Hardware configuration is: Pentium D 3.4Ghz, NVIDIA GeForce 8800 GTS 640MB, 4GB RAM DDR2 800, XiFi Soundcard, 3 Harddrives, 1 DVD Drive. Powersupply is a 550W.