Replying to Quicktime Problem

I have windows XP, Quicktime worked fine on my computer for quite awhile but recently whenever i try to play movies it comes up blank (it still plays sound. I tried restarting my computer and right after i clicked restart the movie showed up and played (until the computer started to shut down). When I restarted it the movies worked fine til i logged out. Does anyone no why this is happening or how to fix it cause it is getting super annoying. Thanks