I have downloaded a dvd from itunes but it is really jumpy.

I have download a dvd from itunes and paid for it but it is really jumpy and keeps intermittently pausing. Has anyone else had this issue. Im not very computer literate so don't really understand why. My computer has played other downloads before so im not sure what is wrong with this one. Any idea what I could do to fix this as im abit annoyed as I've paid for it now.

windows vista computer