GHacks.net posted a guide about downgrading Internet Explorer 9 to Internet Explorer 8

Windows Vista users who have been testing the beta of Microsoft’s latest web browser Internet Explorer 9 may be in for a surprise if they upgrade their Vista installation to Windows 7. What happens is that these users will notice that Internet Explorer 9 is no longer installed on their computer system. Instead, Internet Explorer 8 is once again on the system. In short, the upgrade from Vista to Windows 7 will downgrade Internet Explorer 9 to Internet Explorer 8 if present on the system.

  Vista To Windows 7 Upgrade Downgrades IE9 To IE8