In case you doubt this number, here's what Brian Krebs had to say about his research methodology.
...individually contacting nearly all of the security researchers who submitted reports of critical flaws in Microsoft products to learn from them not only the dates that they had submitted their findings to the company, but also any other security trends or anomalies they observed in working with the world's largest software maker.Additionally, he also shared the data with Microsoft before posting it on his blog. This chart shows all the vulnerabilities that Mr. Krebs included in his findings.
So if you were an Internet Explorer user in 2006, chances are you opened yourself up to compromising your PC. Mr. Krebs reports that the second most popular browser, Firefox, was only exposed for 9 days -- that is 2% vs. 78%. Of course I wish the 2% was lower, but it is clearly much better than Microsoft.
Where to from here? Even with Internet Explorer 7 now released, and supposedly safer, are you willing to risk being exposed to more vulnerabilities with a track record like this? I'm not. In case you have forgotten, here's how you get Firefox.