Why does my laptop screen has a waving effect in the display?

Recently I charged my laptop overnight. The next morning, the display looked as if the icons and everything were wavering, sort of like waves within the display.
This is an old laptop - Gateway. Has Windows 2000 on it though.
I tried taking my cell phone away, because I've seen where people say that that can cause display problems, but it didn't solve it. I also tried reducing the color pixals.
Am I simply sunk because this laptop is really old? Do I need a new battery? (I never use it on battery - only plug it in.)
Help?