I think the XBOX (after being modded) can stream any form of media EXCEPT high definition content.

This is a great website for you Spec!

The program you're looking for is XBOX Media Center.

Here's an "official" 'mod' to the XBOX from Microsoft

Here's a 'how-to' on how to build a XBOX Media Center

And finally, here's the Wiki page for the whole process

Many would say that the XBOX media center is a lot more versatile than the integrated media center extender in the XBOX360, but the main thing at this point is...the XBOX isn't powerful enough to do 720p or 1080i (HD) resolutions...