Ah, this is something else I thought of which would cause it to jump, thoguh I thought you did have them installed. You'll also notice moving windows about that it'd jump a bit too.
Go to Control Panel and go to System, Hardware, Device Manager. Look for Display adapters on the list, and if it has a explanation mark (!) or a question mark (?), you know something is up, or if nothings listed there at all, and its gone to the default windows one.
Click on the one listed there and when the window opens up, go to the Driver tab, and then to update driver. Follow the instructions and try and install them again from here.
If nothings listed go to the Control panel, and go to Add Remove hardware, and press to scan. If it finds a display adapter thats not being used, run the wizard to try and find the drivers. Better still, if you know what card you have, run to do a manual search and check through what drivers windows has found on the system and install the one thats for your card
I know it all sounds complicated, but its not if you follow through
Hope this helps