Go to Start > Control Panel > Uninstall a program > Turn Windows features on or off and un-check the box next to Internet Explorer 9. After that, download and install Internet Explorer 8 from the Microsoft Download Center.
Alternatively, navigate to Start > Control Panel > Programs and Features, select View installed updates from the left, click the Internet Explorer 9 entry and press Uninstall. Reboot your computer and the browser will be restored to the previous version.