http://support.microsoft.com/kb/957700/en-us

Steps for Windows XP or for Windows Server 2003
To uninstall Internet Explorer 8, follow these steps: Close all programs.
Click Start, and then click Control Panel.
Click Add or Remove Programs.
In the list of currently installed programs, click Windows Internet Explorer 8, and then click Remove.

Note If Windows Internet Explorer 8 does not appear in the list of installed updates, try the alternative steps for Windows XP or for Windows Server 2003.
Follow the instructions to uninstall Internet Explorer 8.
When the uninstall program is finished, restart your computer.