Although Microsoft is touting Vista as their most secure OS ever, Symantec examined the new Vista networking technology and determined that it had plenty of bugs just waiting to be exploited. Of course, any time new code is written, there is bound to be at least some bugs in it. Symantec did concede that over the long run, it should be more secure. As far as short term goes, it won't be any more secure.
Symantec is the same company that just last week said in a posting on their blog that Mac OSX had not one existing virus threat.