Implicature
Microsoft is weeks away from releasing the MS Windows Home Server Edition.
Its basically a very trimmed down version of Win 2003 (which we Network Admins use for work) but updated with all the features of Win VISTA. It allows you to create home networks very easily and it should allow for basic file sharing and streaming. It will even recognize your Xbox.
http://www.microsoft.com/windows/pro.../features.mspx
This is your best bet for a newbie. Once you get confortable You could try Ubuntu Server but thats trickier, I's start small and work your way up.
If you need help setting up, drop me a note.