you may need the latest codec pack to play it... it might be encoded with a newer codec than you have.
You can search google for:
codec pack
plus also add the operating system you are using, i.e. 2000 or xp.
Then install it and try again.
Thats what I'd try first.
Unless of course, its an mp4 or mov file, in which case you will need to use QuickTime player, which is available for free download off the apple.com website.