Begining third week of April, users still running IE6 or IE7 on Windows XP, Windows Vista, Windows Server 2003, or Windows Server 2008 will get will get a notification through Automatic Update about IE8. This rollout will start with a narrow audience and expand over time to the entire user base. On Windows XP and Server 2003, the update will be High-Priority. On Windows Vista and Server 2008 it will be Important.
IE8 will not automatically install on machines. Users must opt-in to install IE8. Users will see a Welcome screen that offers choices: Ask later, install now, or don’t install.Users who decline the automatic update can still download it from http://www.microsoft.com/ie8 or from Windows Update as an optional update.
Source:→ IE Blog