http://support.microsoft.com/default.aspx?scid=kb;en-us;Q318378

How to reinstall or repair Internet Explorer and Outlook Express in Windows XP

Windows XP and Windows XP SP1: Edit the registry and install Internet Explorer 6
Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
When you try to reinstall the same version of Internet Explorer, you may receive the following error message:
Setup has detected a newer version of Internet Explorer already installed on this system.
Setup cannot continue.
To avoid this error message and reinstall the same version of Internet Explorer and Outlook Express, follow these steps: 1. While you are logged on as an administrator, click Start, and then click Run.
2. In the Open box, type regedit, and then click OK.
3. Locate the appropriate registry subkey, right-click the IsInstalled (REG_DWORD) value, and then click Modify.

To reinstall only the Internet Explorer 6 browser component on Windows XP, use the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed Components\{89820200-ECBD-11cf-8B85-00AA005B4383}
To reinstall only Outlook Express 6 on Windows XP, use the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed Components\{44BBA840-CC51-11CF-AAFA-00AA00B6015C}
4. Change the value data from 1 to 0, and then click OK.
5. Quit Registry Editor, and then install Internet Explorer 6.
6. To reinstall Windows XP updates, visit the following Microsoft Windows Update Web site