If you're about to add SP3 to XP, you should be aware of this

[GMan_PCTT]

The addition of SP3 to Windows XP removes your ability to revert back to a previous version of IE. This was done intentionally due to the fact that rolling back to an earlier version would also replace some of the new SP3 files with earlier 'unpatched' versions of those files.

So, if you want to have the option to roll back to IE6, roll back before you install SP3. Then, update to IE7 again once SP3 is in place. The files that are backed up as part of IE6 will then be the same as those included with SP3 and the rollback option will be forever available to you. This same idea also applies to anyone running the IE8 beta. Roll back, add SP3 and then roll forward to wherever you want to be. If you don't, you'll lose the option of rolling back to IE7 or earlier and be stuch with the beta until it can be replaced with the full IE8 version.