Windows Vista 3 vulnerabilities, 2 unpatched. Most serious (unpatched): Less critical Oldest (unpatched): 2006-12-22 Windows XP 178 vulnerabilities, 33 unpatched Most serious (unpatched): Highly critical Oldest (unpatched): 2002-09-18 Mac OS X 100 vulnerabilities, 7 unpatched Most serious (unpatched): Moderately critical Oldest (unpatched): 2006-11-22 All data from Secunia. Make of this what you will. My opinions: For being such a young OS, Windows Vista is faring surprisingly well in terms of security. Windows XP, as usual, suffers from numerous security holes, many of which Microsoft never bothered to patch. Mac OS X is clearly better than Windows XP when it comes to security, but can't beat Windows Vista. Apple has done a good job of patching the worst security holes, but Apple definitely has room for improvement on this matter.