The Internet Explorer version 8 (IE8) web browser will be rolled out to Windows users via Windows Automatic Update starting this week.
Do you have to upgrade to IE8? No.
The upgrade comes with a "don't install" option, as well as an "ask me later" and "install" choice.
According to the Sitepoint Blog, all IE6 and IE7 users using the Windows XP and Windows Vista operating system will receive an IE8 update notification. It will be marked as “High-priority” in XP and 2003, and “Important” in Vista. The IE8 roll out will initially target a small audience but expand to cover the entire user base over the coming weeks. (I didn't see it on my computer when I manually ran Windows Update yesterday, so it may be a while before you see it on your PC.)
Details are at the IE Blog.