How do I disable XP auto logoff?

My PC (XP Home, SP2) has started to auto logoff. This is particularly annoying because my PC is running my DVR (BeyondTV) in the basement. So, 30 minutes into watching my show, a login prompt appears and I have to go downstairs. I have been using this PC for at least 3 years without this ever happening before.

I have no screen saver, power management is set to always on, the welcome screen is disabled at startup, the user accounts are set to not prompt password, i checked the “control userpasswords2” and i do not have the login box checked….

What else can I try? Could this be a virus?