[Skindred]

Create a simple autoit script.

Something like this (note that this is not actual code, just a simplistic way of making an example);

Code:

If WindowState.Active("XBMC") && WindowState.Active("Boxee") Then
WindowKill("Boxee")
EndIf

If WindowState.Active("XBMC") Then
WindowKill("XBMC")
Launch("Boxee")
EndIf

If WindowState.Active("Boxee") Then
WindowKill("Boxee")
Launch("XBMC")
EndIf