According the to an analysis by a reporter at Washington Post over 77.8% or 284 days in 2006 Microsoft Internet Explorer 6 (IE6) users on the Microsoft Windows OS platform was vulnerable to malware and exploits, which were designed to steal personal data off users' computers.
In comparison FireFox browser was only vulnerable for 9 days in 2006 or 2.5% of the time.
Microsoft claims that IE7 is much more secure but when it released in November 2007, exploits and malwares were immediately available.
SANS Institute has a chart that list all currently unpatched Microsoft vulnerabilities.
So, if you want to be secure, move away from using Microsoft products. At least until Microsoft gets their act together.
BTW: this is not news as security vulnerabilities had existed in various Microsoft products for many years. The difference is that most technical people, who are not blindly sold by Microsoft marketing, had realized this for years.