From the “better late than never” department:
If you’re one of the last people that prefers IE 6 to IE 7, you might want to tread carefully when it comes to installing SP3 for Windows XP on your computer.
If you have IE 7 installed before the SP3 upgrade, you will not be able to go back to IE 6 once SP3 is installed. According to this article, this is “because the service pack includes newer versions of the old browser’s files. If Microsoft had allowed users to revert back to the pre-SP3 version of IE6 – the one saved on users’ PCs when they upgraded to IE7, and what was used until now to back out of the newer browser – Windows would have ended up in a ‘mixed file state’. This state is not supported and is very bug prone. To ensure a reliable user experience, [Microsoft] prevent[ed] this broken state by disabling the ability to uninstall Internet Explorer 7”.
If you want to retain the ability to uninstall IE 7 from your computer, remove IE 7 before installing SP3, then reinstall IE 7 once the Service Pack update is complete.