P1 - we DO need some more info if we are to come up with anything useful. As ever, PLEASE post:
Operating system
Any 'changes' you have made since this function LAST worked
Have you tried playing a file of the same type (eg .avi/.mpeg/.wmp etc) from your hard drive? If so, what happens?
I had a similar problem a while ago which was down to incompatibility between (new) video card and processor.
As ORAC says, post the link (if you dare
)