Every process in Windows has a process ID; open Task Manager, switch to the Processes view and make sure the PID column is enabled.
There's a kill program on MSDN. What I don't know is what it does internally; maybe it calls a system kill() library, or maybe it enumerates windows and finds process ID's (instances) from the main window.

http://technet.microsoft.com/en-us/s.../bb896683.aspx