To uninstall Internet Explorer 8:
- Open Programs and Features by clicking the Start button, clicking Control Panel, clicking Programs, and then clicking Programs and Features.
- In the left pane, click View installed updates.
- Select Internet Explorer 8, and then click Uninstall.
- Follow the instructions on the screen to uninstall Internet Explorer 8.
- When you're done, restart Windows.
If you don't reinstall Internet Explorer 8, your computer will revert to Internet Explorer 7 and the Internet Explorer 7 cumulative updates that were on your computer before installing Internet Explorer 8.
Run Windows Update to be sure your computer isn't missing the latest Internet Explorer 7 updates.
http://windows.microsoft.com/en-us/windows-vista/uninstall-internet-explorer