Microsoft XML 6.0 Microsoft Windows Installer 3.1 (KB907265)
BITS 2.5, a service used by Windows Live OneCare and Microsoft System Center Configuration Manager 2007 (KB923845)
WPA2 Wi-Fi Security, the strongest form of WPA security (KB893357
Peer Name Resolution Protocol 2.1, to enable XP SP3 programs that use PNRP to communicate with Vista programs that use PNRP (KB920342)
Other bundled updates include changes to IPSec virtual private networking and a new service for logging into domains (DIMS).
SP1 Users, Rejoice! SP3 Includes SP2 (and SP1)!
Windows XP SP3 includes both SP1 and SP2, so if your system's been under a rock and missed SP2 (or if you're doing a fresh install from an original Windows XP or XP SP1 disc), you can install SP3 anyway. However, you must have SP1 installed to use SP3.
Here's What New in SP3 (Not a Whole Lot, to Be Honest)
SP3 adds the following new features:
"Black Hole" router detection is enabled by default
Support for Network Access Protection (NAP), a feature in Windows Vista and Windows Server 2008
The Security Options dialog in Control Panel now includes an "Explain This Setting" tab to help keep you from making security setting goofs.
Enhanced security for Administrator and Service policy entries in System Center Essentials
New kernel mode Cryptographic driver
No product key needed for SP3 installation; it's requested after installation
Microsoft's been warning XP SP2 users that SP3 wasn't going to include a lot of new functionality - and they were right. Most of the changes in SP3 are not going to be visible to end-users.
I've Got XP x64! Where's My Update?
Windows XP SP3 is for 32-bit versions of Windows only. x64 users should install Windows Server 2003 SP2, which has been around awhile (over a year, in fact).
For More Information
As usual, Microsoft has lots of documentation you can grab, including