The Security Intelligence Report (SIR) is a comprehensive study of malicious software that Microsoft releases twice a year.
The latest report is Volume 8 (SIR v8) and covers analysis from September 2009–March 2010.
SIR v8 shows that the nature of cyber threats has become more sophisticated and professional but so has the ability of security professionals and software to protect against them.
Also, incidents involving attacks (hacking, malware, and fraud) have been significantly outnumbered in recent years by incidents involving negligence (lost, stolen, or missing equipment; accidental disclosure; or improper disposal).
The report finds that newer operating systems like Windows 7 are more secure. However, no matter what operating system or software your computer is running, the best way to protect it is to automatically install security updates.