From Jan2005 to Dec2005 is a year-end summary of software vulnerabilities.
There were 5198 reported vulnerabilities: 812 Windows operating system vulnerabilities; 2328 Unix/Linux operating vulnerabilities; and 2058 Multiple operating system vulnerabilities.
Notice the WindowsXP is at the top with the most. Also with the Critical IE vulnerabilitie.
Windows XP 77.92%
Windows 2000 9.82%
Windows 98 4.78%
Mac OS 4.11%
Windows ME 1.99%
Windows NT 0.86%
Linux 0.30%
Windows 95 0.12%
Web TV 0.03%
Windows CE 0.02%
SunOS sun4u 0.01%
PSP 0.01%
Hiptop 0.01%
FreeBSD i386 0.00%

Theres a reason i love FreeBSD not sure about anyone but thats just me. one OS that i like along with SuSe