How can i initiate a shutdown (CTRL+ALT+DEL) without using JNI on my Windows box?

You can call Runtime.exec() and execute rundll32.exe with a few parameters.

rundll32.exe shell32.dll,SHExitWindowsEx   (to close the session)
rundll32.exe shell32.dll,SHExitWindowsEx 2 (to reboot)
rundll32.exe shell32.dll,SHExitWindowsEx 3 (to power down)