I had the same problem, you must change the Avast settings to get Windows Media Center to work.
1. Start the Avast interface page.
2. On the left side, click "Real Time Shields"
3. On drop down menu, click "Behavior Shield"
4. On Behavior Shield page, click "Expert Settings"
5. On Behavior Shield Settings page, click "Trusted processes"
6. On the Trusted processes page, browse for the file "C:\Windows\ehome\ehshell.exe" and click "add" and then "OK"