What do you mean "capture key down event in the window service" ??
you mean while running a windows service, you want to capture the key
press event or what ever the key is down???
If so, you are trying to write a keylogger, (umm... trying to hack
something?) which would be blocked by many of the anti-spywares. I
mean to say that would not be a good idea to implement in an
application which is not a spy..