Microsoft’s Internet Explorer development team has warned users updating to Windows XP Service Pack 3 that they won't be able to downgrade from IE7 to the older IE6 without uninstalling the service pack.
"If you choose to install XP SP3, Internet Explorer 7 will remain on your system after the install is complete," said Jane Maliouta, an IE program manager, in the blog entry. "Your preferences will be retained. However, you will no longer be able to uninstall IE7. If you go to Control Panel, Add/Remove Programs, the Remove option will be grayed out."
See here please from HardOCP.