I've downloaded a number of vids. They all play but you can only hear them, not see them.

This doesn't happen with the odd vid, it happens with them all.

Any solutions, guys?

yes ... most likely CODEC problems. I'd highly recommend getting a back up player even if you do update your codecs. Such as VLC which has in-built codecs; CODECs list for Movies/Audio/ Alternate players / Video help forums