I remember trying doing something similar not long ago, but it turned out that you can call a winforms app from a windows service but it starts as a hidden process and the GUI never gets rendered (you can see the process in TaskManager). The key point to have a windows service to begin with, is so that they can run separately to Windows forms app. However, if you do get it working, please share...
Also have a read here :
Winforms-Windows service[
^]
Cheers...