The third service pack to Windows XP includes the previously released updates and hotfixes to Windows XP, creating a new baseline for servicing.

If you wish to prevent users from installing Windows XP SP3 through Automatic Updates,
download and deploy the Windows Service Pack Blocker Kit.

Information about SP3 | Download the Release Notes

Some users have reported loss of some customizations such as themes, visual styles, etc. and
found this over at ZDnet:

"It totally un-installs IE7 (at least it did on our image).
It requires that you do a complete re-install of IE7.

11) It kills a lot of the special settings we had made in HK_Local_Machine.

12) It re-sets the User profiles and causes a lot of re-setting to be made of the user interface (Explorer)

13) It forces new settings to be made of Windows Automatic Update and the Firewall.

Good luck....hope to hear your success reports.