Locking screen resolution

Is there anyway to lock a screen resolution? Perhaps in the registry or something? I have a PC running WinXP with an onboard video card. Everynow and again it will just change the resolution when I do a reboot. It can stay fine for a few days but then next time I start it changes to 800×600. I have no applications that I can think of that would change it and I have logged in as administrator and set the resolution there to what I want.
Please can someone tell me a way to lock the resolution so nothing else can chage it. Thanks.