Pere Gurevich, one of the IE program managers, talks about the use of ClearType in Internet Explorer 7:
You may have noticed that after installing IE 7, your fonts in IE and Outlook Express look different. That is because IE7 has changed our text rendering to use ClearType. I hope you like the change. If you are not familiar with ClearType, you can find more information on it using the link below, but basically it is another method for rendering Text on LCD Monitors.