Executables have to expose several functions to be recognised as a screen saver by Windows:
ScreenSaverProc
,
ScreenSaverConfigureDialog
,
RegisterDialogClasses
- the full details are in
scrnsave.h
I don't know if this is possible with Flash, you might be better off writing (or searching for) a wrapper in C or C++ that calls the Flash movie.
Here's one that came top of a Google for "Flash screensaver creator":
http://www.instantstorm.com/[
^]