@ Vunfah: To answer nike1's question: yes you can download the single codec and install it :)
Here is some more info about codecs.
You can download the codec here (mind you I haven't tested this - I'm just reporting it, since my own codecs work ok and I don't want to mess with them. Had serious trouble because of a codecpack a year ago and ScottW spent quite some time helping me fix it).
The site has a forum too, so if WMP doesn't find the codec after installing you can ask them for more help too.
I'd also install VLC player as Whs suggested, simply because it is a wonderul player that will allow you to play just about anything. I've had corrupted mp4 files that Vlc player will play, whereas other apps just won't.
I found that a lifesaver, because now at least I could see if there were important files amongst them and even if no other app played them, at least I could see them ;)
(Sorted it eventually, but that is another story ;D).