On Windows XP this will only work till you have not installed the SP2. Once you install SP2, it will automatically redirect it to v5 of windowsupdate.

Do a reboot. Remove the update activex control from the following directory:

And remove the following directories:

And from the C:\Windows\System32 remove the wuweb.dll file.

And then go to the http://v4.windowsupdate.microsoft.com/en/default.asp, where the site would reinstall the v4 activex component. After it is installed, make sure you reboot the system to get all the files properly installed.

Update: It seems that Microsoft has now made the V5 as the default version for the Windows XP even for systems that don't have XP SP2 installed. So I guess, this post is irrelevant.