You probably have a problem with CODECs...
Have a look in this Tips & Tricks forum for other suggestions about CODEC Packs.
For me, I recommend >
Cole2k Codecs
Just click on the link, download the STANDARD pack and install it using the EASY method only (You don't need the toolbar that is offered).
After installing any CODECs pack you should immediately shut down your computer.
After you start again you will almost certainly be able to watch almost anything !
Let us know how you get on !