Try using a different player, Windows Media Player for example. Players are not (unfortunately) universially compatible as each are made for their own format and as a second importance try and run the other guys format.
I've never been impressed by Real Player even for Real Player files! If Tontoman's suggestion doesn't work it could simply be that your processor/graphics card aren't up to scratch or that you have too many programs running. (I usually have loads of programs open simultaneously and suffer poor playback until I close a few of them).