If can do it if you throw out all code related to
System.Windows.Forms
. If your UI is well isolated from everything else, this is easy. If not, try to extract pure application logic, data processing, etc., everything not related to UI. If this is difficult, you better start from scratch, using your available application as a reference. Next time think thoroughly about isolation of different application layers.
Development of a Windows Service itself has a number of special considerations and difficulties, especially in testing/debugging.
Start here:
http://en.wikipedia.org/wiki/Windows_service[
^],
http://msdn.microsoft.com/en-us/library/d56de412%28v=vs.100%29.aspx[
^],
http://msdn.microsoft.com/en-us/library/y817hyb6.aspx[
^].
—SA