CNET: Windows 7 almost five times more secure than XP
Those are the findings of Microsoft's latest Security Intelligence Report, which detailed in depth the state of software vulnerabilities, exploits, security breaches, and malware in 2010.
http://news.cnet.com/8301-1009_3-20063220-83.html#ixzz1MXpezMKz

Microsoft SIR Report - Volume 10
Direct download (PDF) or Microsoft webpage:
http://www.microsoft.com/security/sir/

What is SIR: The Security Intelligence Report (SIR) is an investigation of the current threat landscape. It analyzes exploits, vulnerabilities, and malware based on data from over 600 million systems worldwide, as well as internet services, and three Microsoft Security Centers.

Regards
Urs