Is it a laptop or desk top?
What Operating system {OS} does it use- Windows XP , VIsta , or Linux etc?

if desk top there should be some buttons on the monitor that you can use to center and align the screen - there might be a auto align button that you can push to fix it.

If that doesn't work or isn't available -go to the control panel and display settings section.

I guess I'll wait to get you answer about the OS before going on more...