Screen freezes in XPPro after inactivity

Never while I’m actively using the computer, but always after a period of inactivity (? an hour), the screen/computer locks up, and I can’t use the mouse, keyboard, ctrl-alt-del, etc. to get out of it. I must hit the restart button. Doesn’t matter which user I’m logged in as, or even if it’s still sitting at the login screen. Running XP Pro on a Via EPIA-M Mini-ITX Mainboard. I’ve set the prefs so the monitor, hard drive, and computer never sleep or standby. I’m using an 802.11g USB network adapter, but that seems to be working fine. Please help!