In the second example (in the section examples) on this link, there is a description on using WM_QUERYENDSESSION to abort a shutdown. It also states that this does not work on versions of windows later than XP. This is conflicting with the advice given on another question here at stackoverflow. What is the correct answer? I do not have a computer with either so I am unable to test.
Some applications got the WM_QUERYENDSESSION handling wrong (Not passing to DefWindowProc, they incorrectly returned 0 even though they did not intend to block shutdown) and so MS changed it with Vista, you now need to call ShutdownBlockReasonCreate()
@Konamiman: shutdown.exe -a will abort a "scheduled" shutdown yes, but not a "normal" shutdown by someone calling ExitWindowsEx()