If you use p2p to download you nead to have the beginning of the file and the end otherwise it won't play.Even a preview. It also depends on how much of the file you have got, in my experience. I haven't found a freeware tool that will fix mpegs that works yet but Divfix works for Divx AVI files quite well.

Heres the link for it: http://divfix.maxeline.com/