... to use Windows Vista instead of any older Windows version, then it's this one:

Microsoft Malicious Software Removal Tool (MSRT) data shows that the infection rate for Windows Vista–based computers is 60.5 percent less than that of computers running Windows XP SP2, and 91.5 percent less than the infection rate for Windows XP with no service packs installed.

From the MS Security Intelligence Report July-December 2007. The numbers are normalized, i.e. they take the different number of installations of each OS into account. They are even more impressive when you consider that the number of PCs running Windows Vista has increased by millions during the last two years.

I'm pretty sure that this is mainly because per default users on Windows Vista aren't administrators. Most of today's malware assumes that the user has administrative rights.

You should read the whole report. It's somewhat depressing, however. According to the MSRT data, the authors of the "storm worm" (Win32/Nuwar) created a botnet of about half a million systems, estimated. And

In 2H07, the MSRT removed malware from 15.8 million distinct computers worldwide, an
80 percent increase over the first half of 2007

And Microsoft Malicious Software Removal Tool is far from perfect in detecting and removing malware. It's biggest advantage is that it runs every month on all Windows PC which have automatic updates enabled.