Try removing and reinstalling your video drivers.

Right click on My Computer, then choose Properties. Then, click on the Device Manager tab. Under there, click the '+' symbol next to display adapters. Highlight your display adapter device, and then hit the <Delete> key. Restart your system. If it doesn't automatically reinstall the video driver, you'll need to either download the driver, or install it from a CD you were probably provided with. After installing the driver, you'll need to restart to get your resolution set correctly.