I built this computer earlier this year (early spring), and it has been running fine since (i had absolutely no problems with the initial build). Now, however, it has started having trouble staying on. It will randomly turn off, often quickly after i turn it on (before boot or even POST, or during one of these processes). I am suspecting the power supply, but there are a few odd pieces to the situation.

I know it isn't the CPU or RAM (if it were either of those, it woouldn't boot, right?), and i doubt it is a motherboard failure (same). I have been able to boot successfully into windows, and mostly everythign seems fine. The only exception is that i have no network connection (which could easily be unrelated), and attempting to run 3D games (i have tested HL2 and Crysis) will cause it to shut down. When it turns off it issues a single beep, no matter where the computer was in the boot process or what i was doing.

So I am strongly suspecting the power supply, but the fact that specific (graphics intensive) applications cause shutdown also points to the graphics card. The beeping bothers me, because it seems to say that the computer has some idea what is wrong, but can't tell me. Any help with this is much appreciated, it was an expensive machine that i was hoping not to have to mess with or replace for a very long time. I can get detailed system specifications if they're needed.