[Note that I'm going to be revising this article in the near future, as it doesn't reflect the whole picture]
Inevitably when discussing the merits of Open Source Software (OSS) vs. propritary software, the (in)security of Microsoft products is brought up. I've gathered vulnerability numbers from Secunia, and did some non-scientific analysis on them.
I compared vulnerabilities in 2005 for operating systems (RedHat Advanced Server vs. Windows 2003), web browsers (Firefox 1.x vs. Internet Explorer 6), e-mail products (Thunderbird vs. Outlook 2003), databases (PostgreSQL 8 vs SQL Server 2000), and office products (OpenOffice.org 1.1 vs. Office 2003).