Well i got my PC about a month or two ago brand new and been using it mostly for gaming. Keep it well ventilated and dust free. But the last 3 days its been rebooting itself for no apparent reason usually 1-3 times a day. Thought it maybe dust or a over-heated issue but its pretty cool and hardly no dust, so then thought it occured after a while of playing a game (Counter-Strike: Source) but ive left the computer idle a few times to come back and it has rebooted. Wondering if this could possibly be a failing PSU or what else if not??
Also about a week after getting the PC the PSU fan was making strange grinding noises but that passed after i few days so thought nothing of it.
If you suspect temperatures, use HWMonitor (download from www.cpuid.com ). It also shows voltages detected in the 3.3, 5, and 12 volt rails. If it is your PSU failing then those voltages might be fluctuating too much.
If it is only a month or two old I would contact who you got it from and have them look at it.