Media Center does not want you to watch E4 :-)
 

 The TV channel 'E4' is not detected during a digital service scan on Media Center 2005

 http://support.microsoft.com/kb/555330/en-us

 

Basically, when Media Center 2005 was released, E4 was encrypted.  The nice people at Microsoft, to prevent you from seeing a channel you couldn't access, excluded it from view.  Problem is time marched on and E4 changed.  Media Center didn't know.  Eventually, Media Center was changed via Update Rollup 2 for Windows XP Media Center Edition 2005.  But you still have to manually rescan to add the E4 channel at that point..

Or you can follow the directions for registry tweaking :-)