Theres loads of examples of screen capture on the web:

http://www.developerfusion.co.uk/show/4630/

....... but this is not the kind of work you would do from a service.  A
service should always be build assuming there is no logged in user and it
can run in the background, therefore no window to capture from.

You should probably instead create a program that starts up at login that
captures the screen if instructed to do so by the service - a simple socket
call should allow communications between the two.

Regards

John Timney (MVP)