As WMP11 ships with Vista changing to v10 might be problematic. A quick Google reveals some ways to achieve it but it looks like a bit of a hassle.
As for it not playing videos, that's more likely a lack of codecs than a problem with WMP. Try the KLite codec pack, Standard is usually sufficient. click here
If you want to try a different media-player;
GomPlayer; click here - Nice interface and plays lots of file-types without needing additional codecs
VLC click here; Ugly looks and unpolished interface but plays almost any file (although you can apply skins to improve the looks)