One thing, will not play all formats or only a certain format?
well, u can try another version of the video driver (a lower one)
But first try a different player from the bunch that's out there.
And try updating your codec pack. I don't know what codec pack U use. I use K-lite codec pack.
(U can Enable all filters, detect boken, bla,bla,bla... to try and solve it)