What causes vertical lines on computer monitor?

Either the monitor or video card are bad or the wrong software driver is being used for the video card or the refresh rate is out of range for the monitor. How to resolve it: Plug in a different monitor swap the video card into another PC or try a new one make sure the correct software driver is being used for the video card.

Or there is simply too much dust in the video card fan. Q-tips may be the solution.