Well you would need access to the current windows session ... You could do
this either by allowing interaction with the desktop (beware security loop
holes such as a shatter attack) or by going through and actually connecting
to it. I don't have any C# code for the latter handy but I will poke around
for some.

Since this type of application is obviously geared to be running in a user
context I might ask why you need to run it as a service.

Cheers,

Greg Young
MVP - C#
http://codebetter.com/blogs/gregyoung