You need to go to "Installed Updates" to unistall IE9 beta in Vista or Windows 7
Here are the steps to uninstall Internet Explorer 9 Beta:
1. Go to: Control Panel -> Programs -> Programs and Features -> Installed Updates
2. Now scroll down and find the "Microsoft Windows" section.
3. Then locate "Windows Internet Explorer 9", right-click on it, and then click Uninstall, and then, when prompted, click Yes.
4. You will then need to Restart your computer to finish the unistall