http://www.vistax64.com/vista-general/75750-vista-does-not-allow-me-save-c-drive.html 

well I have spent about an hour making sure i am owner and full control.  but it still will not save a render to screen.  still no dialogue box opens for that...it is really strange because it opens for scene object...etc...just not for picture.  havent tried animation yet...but i found a very disturbing thread.  Please see the link above.