I have a Windows XP machine that locks itself after an unknown amount of time. I have disabled the screen saver completely, and unchecked "lock computer when coming out of standby". I've even overrode the settings with gpedit. Even still, it locks itself after a while, and it doesn't take long.
If it's any sort of hint, the time period for the screensaver is greyed out, even if I choose an actual screensaver.