Help Please!

Hey everyone I just put together my first computer and everything was working great. Second day in the monitor shuts down mid use, its almost as if its sleeping, but i cannot get it back up, the computer is still running, and everything in the tower seems to be running, just the monitor shuts down. Now it seems to do this every 20 or so minutes. I tried the monitor from our other computer as well with same effect. Shuts down in use sometimes and sometimes when idle over night does the same thing.. Let me know if you can please help or no any possible reason for this...it was suggested that this may be my power supply but i'm not sure that only the monitor would shut down

I'm running:
Windows Vista Home Premium
AMD Athlon 64X2 Dual Core Processor 5200+
NVidea Geforce 8800 GTS
2.0 gb ram
PSU Xion 500 watt

monitors are Optiquest-Q9 and Viewsonic VA 520 both have same problem


thanks again

haha wish that was it, all sleep/standby/hibernate modes are turned off, monitors work fine on other computers so makes me believe this is either problem with vista, video card, or psu

thanks for the tip, i tried what you said caps lock light does not respond, so i guess system has crashed, how do i test memory and what do i do next? ram is 2 1gb ddr2 corsair...thanks again