There are a couple of ways of integrating Boxee with Windows Media Center and now there is another neat solution. Developer teknowebworks.com has created a small installer that adds a Boxee tile to Windows Media Center, clicking on it launches Boxee and when you quit Boxee it returns you to Media Center.
All you need to do is download the MSI file, install it and then you will have an easy way to launch Boxee from Windows Media Center. I use Media Center most of the time but there are occasions that I switch to Boxee so its great to have an easy way to switch between them
You can download the file for free from teknowebworks.com