When Microsoft Internet Explorer 6 (IE6) was released in 2001, it was cutting edge. However that was nine years ago.
In the nearly decade since then, Microsoft has released two updates to the browser, and now has its eyes set of IE9. Still, in their 'State of the Web Q1 2010' report, Zscaler found IE6 still held more than a quarter of (26.93 percent) of the browser market share in March. That number was actually a slight decrease from January, when it stood at 33.46 percent.
The problem, Zscaler points out, is that IE6 lacks some of the security protections found in more current browsers, such as IE8. For example, the report notes, "it is not uncommon for new vulnerabilities, such as a recent 0day (CVE-2010-0806), to impact older browser versions, such as IE6. This same vulnerability did not affect IE8."
The drop off in use between January and March seems to be due to enterprises upgrading to IE8, which saw its uptake increase from 5.79 percent of the market to 10 percent. Part of that is likely due to highly-publicized incidents such as the infamous Aurora attack that impacted older versions of Internet Explorer.
More current browsers, from IE8 to Mozilla Firefox 3.6 include a number of enhancements designed to improve security. The message here - upgrade.