After further testing, it seems it's the files that are the problem. There's no problem with YouTube HD. There's no problem with playing BlueRay discs and ther'es no problem playing high end games. The files I'm having trouble with are .mov files that I downloaded from Footage Firm. I've downloaded both SD and HD files from them but only the HD files are giving me trouble.
I wonder if it's possible they got corrupted on the download as I have a weak internet connection with Hughes net satelite.
Someone else suggested using "integrated graphics". I don't know what that is.
Thanks to everyone for your replies and suggestions.
I still don't know how to determine if those .mov files are corrupt.