computer freezes after 15 minutes of inactivity

I have a wierd problem. I just built a new PC. Here are the components:

MSI P6N SLI Platinum mobo
EVGA GeForceGTS 320 MB
PC Power and Cooling Silencer 610 EPS12V
Intel Core 2 Duo E6850
Kingston Value Ram 2GB (2 x 1GB)
SAMSUNG SpinPoint T Series HD501LJ 500GB
mx518 mouse logitech

The PC works great for hours on end when I am using it. If I go away from the computer for more than 15 minutes then the screen will become sort of unresponsive. I will be able to move the mouse briefly but I can't close/open windows and then it basically freezes shortly thereafter. The clock does update during this time though.

The only time it didn't freeze when away from the PC for more than 15 minutes was when I left the Medal of Honor Airborne Assualt demo running. Leaving winamp running a song didn't keep it from freezing.

I have turned off/on the screen saver with no change. Switching the power options around like telling the monitor or harddrive to always stay on has not helped.

I'm not sure what to do. There is one thing new to me in the system tray. The safely remove icon shows up (like the one used for USB flash drives). These are the 1st 2 SATA drives I have used so maybe that is standard??? I have all the latest XP critical updates and SP2. I have the latest video card driver. I have the latest BIOS.

The PC plays games flawlessly but this freezing problem when away from the inactive PC has me baffled. Please help. Thanks.

I am not seeing any errors or warnings. Event viewer isn't catching anything.