Guessing its Winxp os and you have up-to date graphics adapter drivers?(What make of adapter do you use) Maybe a bit more info on sys.spec.
It can happen because of game playing (depending on the game)or sometimes when you have done a systems restore,though in theory it should not.Had the same happen last night after setting up MSN Cleartype program.Believe Java was to blame then but i do change and alter my Display settings and appearance alot but save the file so it loads at startup/reboot.
If running winxp have a look in the event viewer via my computer/manage, computer management to see if applacations, or systems,log shows any error at times.(red cross )
Its a problem, but why as you say( learn't to insert the VGA disc etc.)makes me wonder if you have ever had graphic driver updates? Here from you iam sure.