http://www.theregister.co.uk/2011/09/28/window_malware_infection_exposed/

Java, Adobe vulns blamed for Windows malware mayhem

Five products hit in 99.8% of hacks

Failure to patch third-party applications has become the main reason that Windows machines get infected with malware.

….

Systems running vulnerable versions of Java JRE, Adobe Reader and Acrobat, and Adobe Flash were particularly at risk of attack. Vulnerabilities in Internet Explorer, by contrast, only cropped up in 10 per cent of attacks. By contrast, Java flaws (37 per cent), Adobe Reader/Acrobat (32 per cent) and Flash (16 per cent) were far more productive routes towards exploit