A couple of useful links:

http://www.any-video-converter.com/products/for_video_free/

Basically does what it says on the tin. Converts pretty much every format to what you want for free. Very useful for some formats that Media Centre does not always recognise.

http://www.techcrunch.com/get-youtube-movie/

This link will download any You Tube clip by dropping the copied url in to the box. It creates a flv file which Media Centre sometimes converts - if not use any-video-converter first.

Needles

great links! thanks needles