I have xp. When I try to run certain games, I get an error saying, "unable to create multimedia textures. Have you installed OpenGL "? Please help!!

See Programs that Require OpenGL Support May Not Run in Windows XP and then verify if your video adapter supports OpenGL:

If not, upgrading your video adapter to one that's supports OpenGL may be required.
If so, updating the drivers may be all that's necessary. Drivers can be found on the video adapter manufacturer's web site.

There may have also been an option when you installed the game for OpenGL support.