Microsoft Trustworthy Computing team have recently published the Microsoft Security Intelligence Report (SIR) which provides information about the changing global threat landscape of exploits, vulnerabilities, and malware.

This special edition of the SIR provides summarized information about how malware (and other forms of potentially unwanted software) has evolved over the last 10 years:

You can download either the full report or the summary report here: http://www.microsoft.com/security/sir/story/default.aspx#!10year