Computer lockup/power off/??? problem

I don't know what to call this problem really. I just built a new computer and it worked great for a couple weeks, but now if I let it sit long enough(ie I'm away from the computer doing nothing), the monitor stops recieving a signal, the fans turn off, the mouse optical light turns off, but the power and CD/DVD drive lights stay on. Is this a PSU problem? An overheating problem? I have no idea what is causing it! Could even be software, dunno. I'm running ABIT's uGuru utility, so if you need to know fan speeds or temperatures, I have em(seem to be very normal). I've scanned for viruses and spyware already. Help!

AMD Athlon 64 X2 3800+
Enermax Noisetaker 485 Active PFC
Abit AN8-Ultra
ATI Radeon x800 XL PCI-E
1 gb ram
1 SATA HD, 2 ATA HD
DVD/CDRW combo drive
CDRW drive
Windows XP Pro SP1

Sound like it is going into sleep mode., and not waking up properly. I have had the same problem in the past. In the BIOS there is a energy managment settings S1, S3, S5. And some thimes timmers for HD and video. Turn off when you find it. Windows is controlling the same hardware in the Screen Saver/Energy Settings.

Check to see what windows settings are set too. The default setting are Home/Office with 15-20 min timers.

I just put together the computer 2 weeks ago, so there's no dust in it yet (got the Lian Li PC60 Plus case, awesome air filter for the intake fan).

I'll check the rest and get back to you guys.