Open the registry as administrator.
Depending on the version of Internet Explorer, search the appropriate registry subkey, right-click on the IsInstalled (REG_DWORD) value and then click Edit.
To reinstall Internet Explorer 6 on Windows XP, use the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed *Components\{89820200-ECBD-11cf-8B85-00AA005B4383}
To reinstall 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}
Set the value to 0 and then click OK.
Close the Registry Editor, and then reinstall Internet Explorer 6 or Outlook Express 6.
You can also fix this error by running Windows Update: