pc shuts down but power light is on

I have a compaq EVO D310 with XP pro. Recently as I am working or the pc is sitting, it will automatically shut down. It isnt powering off because the lights on the MB and the power light on the front of the case is lit. The fans run. The keyboard light goes out and cap/num locks do not respond and stay un-lit. The screen goes black and the monitor light goes orange. From here I power down by unplugging. SOmetimes it comes up sometimes it doesnt. I have swapped HD and Ram. Still happens. What are recomended testing procedures. I ran diagnostics and got a random data test failed memory compare mismatch but when I ran it again this passed and ran fine then the next day it went down again..