Like this ?
There is two ways (as I know ) to do it.
First, add the AVI movie to media center library. To do this , opem media center, Click on movies

Now right click on the screen, and select manage libreary

Click add folder > next > on the computer > next .... and browse to the avi. Add it .
Otherwise, right click on the avi file, select properties. Change open with option to media center. if media center is not in recommend programs list, browse it in other programs, and if it is not in other programs too, browse it to your C:\Windows\ehome\ehshell .
By diong it , media center is the default program to open avi files. If you dont want it to be the default, just be there in open with list, change the open with option to WMP or VLC in the same procedure described above. Media center will still be there in the right click option, as in my shot, where WMP is the default, bet media center is still there.
Hope it helps.