After Internet Explorer 7.0 was released in October it has quickly climbed to claim a 25% market share in the US (largely due to it's rollout via Windows Update). This makes it the second-most used after IE 6.0 in the United States, according to Microsoft's IE Blog. Microsoft claims more than 100 million users have installed Internet Explorer 7.0. By contrast, Firefox's U.S. market share was slightly below 16% six months ago (though it likely has increased since then).
Look for IE7's market share to continue to increase with the retail launch of Windows Vista at the end of this month.
Read more at The Tech Report