(The original, classical definition of this term is "to throw something through a window". )
2- to switch from MS windows to Linux, Mac OS, or another operating system. As in 'throwing' yourself away from Windows.
3- bluescreen of death. As in the user is thrown out of Windows.
2- I defenestrated to Mac OSX recently. Screw M$
3- My computer crashed and defenestrated me. Again.
Bobby: "What can't I defenestrate my trash out on to the highway?"
Helga: "Are you retarded?! That's called littering, we could be fined!"
(Cue the police)