For both of you guys see Links below.
It could also be a codec problem. You may want to try
VLC media player, it has the codecs built in and will play most file types. More info on codecs and other players in this thread:
CODECs list for Movies /Alternate players / Video help forums.
Or it might be that the files you've downloaded are fake. The network is full of them, but you can learn to avoid them and find good ones. Look up the files with
Bitzi before downloading to help sort out the bad ones.
See:
Recognising Fake files
And for tips on finding good files, see:
How to find music..
