Check out the
90-day report card (.pdf), stacking up flaws reported and fixed in Vista against vulnerabilities covering during the first 90 days of Windows XP, Red Hat Enterprise Linux 4 WS, Ubuntu 6.06 LTS, Novell SUSE Linux Enteprise Desktop 10 and Mac OS X 10.4 (Tiger).
Also available a nice article from ZD Net about the first 6 months of VISTA:
