Despite the other rosy reports of superior security for Vista [webmasterworld.com], a study done by Australian anti-virus firm PC Tools claims that although Vista is better than XP, that it's easier to infect than Win 2K.
"...the ThreatFire community has shown that Windows Vista is more susceptible to malware than the eight year old Windows 2000 operating system, and only 37 per cent more secure than Windows XP,"
Additionally, a Sr. Microsoft official claims the malware is adapting to Vista:
Despite all the anti-malware roadblocks built into Windows Vista, a senior Microsoft official is lowering the security expectations, warning that viruses, password-stealing Trojans and rootkits will continue to thrive as malware authors adapt to the new operating system.
Doesn't sound like Vista is much of an improvement in the security department and has been getting kudos for being less vulnerable simply because some of the old vulnerabilities didn't work on the new operating system.
Hackers are adapting, keep your virus scanner current.