debunking myths and facts,
http://www.theregister.co.uk/2004/10/22/linux_v_windows_security/

from the register,
http://www.theregister.co.uk/security/security_report_windows_vs_linux/

"Much ado has been made about whether or not Linux is truly more secure than Windows. We compared Windows vs. Linux by examining the following metrics in the 40 most recent patches/vulnerabilities listed for Microsoft Windows Server 2003 vs. Red Hat Enterprise Linux AS v.3:
...
The results were not unexpected. Even by Microsoft's subjective and flawed standards, fully 38% of the most recent patches address flaws that Microsoft ranks as Critical. Only 10% of Red Hat's patches and alerts address flaws of Critical severity
Also in PDF format here.