Exactly when do these lines occur?
Are they there all the time or just when viewing images or videos? Do you see them when the system is booting or only once Windows has started?
If you have installed the latest video and sound drivers as I suggested the next step is to check the monitor cable is fully connected-take it out and reseat it, use the hold down screws to make sure it's fully home if it's a VGA or HDMI type connector.
If the problem is still there, remove and reseat the video card and memory modules.
If the problem looks like any of these you may have a more serious problem with the card itself:http://www.playtool.com/pages/artifacts/artifacts.html