Having bought a 17" widescreen media centre PC at christmas, I was struggling to work out why the screen had reverted back to 4:3 aspect ratio and how to change it back. This was a tough nut to crack with neither the MCE help, intuition or the Internet turning over many stones (unless I just wanted to be envious about all those people with 50"+ plasma screens). I did work it out in the end though
While watching a TV programme, hit the "Info" button on your remote contol and select the "zoom" option to select which option you want.
You may also need to install the following patch as well, which fixed a problem with the third zoom mode not displaying correctly. It's fairly recent - was posted up on 28th Feb this year. http://www.microsoft.com/downloads/details.aspx?FamilyID=e3ca556b-29f6-4d69-a456-aeb0b92ad57a&displaylang=en