just built a new system, 370 watt psu. all was working great for like 2 days continuously and then came home and found it turned off. so i turned it back on and it immediatly turned back off, before MONITOR lit up or bios appeared. normally i would think overheating or BAD psu, but it was running great continuously for 2 days! any ideas?I think your diagnoses are the most likely. Try reseating every component and try it again. Do you have another working power supply you could test with after that?A fan could have FAILED in the meantime.

Inspect all hardware.it seems to be working again now so i'm gonna watch it in the mean time. all the fans appear operational. i'm tempted to rule out overheating b/c if that were the case then i think playing processor intensive games like doom 3 and what not would KILL it rather than idle. as for psu, its an antec truepower which i believe are reliable, but i'm not ruling that out yet. if it dies again i'll try a new psu. if any other ideas... let me know! thanks!Could be a power brownout in your area.