Windows XP Service Pack 3 (SP3) includes the following:
* All previous security patches and fixes released for Windows XP.
* All enhancements and new features that were included with Windows XP SP1 and Windows XP SP2
* Only two new features have been added: Black Hole Router Detection Windows and Network Access Protection (NAP).
* The following are already available for Windows XP in stand-alone updates. Windows XP SP3 includes them by default (System administrators must choose to install each of these updates): MMC 3.0, MSXML6, Microsoft Windows Installer 3.1 v2, Background Intelligent Transfer Service (BITS) 2.5, IPsec Simple Policy Update for Windows Server 2003 and Windows XP, Digital Identity, Management Service (DIMS), Peer Name Resolution Protocol (PNRP) 2.1, Wi-Fi Protected Access 2 (WPA2).
* From the Release Notes: "Microsoft is not adding significant functionality from newer versions of Windows, such as Windows Vista, to Windows XP through XP SP3. For instance, Windows XP SP3 does not include Windows Internet Explorer 7, although Windows XP SP3 does include updates to both Internet Explorer 6 and Internet Explorer 7, and it will update whichever version is installed on the computer. One notable exception is that, SP3 does include Network Access Protection (NAP)."
* *This next bit is going to get confusing. Microsoft stopped supporting the early releases of Windows XP. You can't apply SP3 unless you have, at bare minimum, Windows XP SP1a. However, Service Pack 2 includes all of Service Pack 1a. So... if you have either Windows XP original (with no Service Pack), you must install Service Pack 2 first, followed by Service Pack 3. If you have Windows XP Service Pack 1a or Service Pack 2 already on your system, you can install Service Pack 3 now.