If you have Windows XP Service Pack 1a or Service Pack 2 already on your system, you can install Service Pack 3 now. In addition to general product fixes, this includes improvements in stability. These updates include two main categories of fixes: Previously unreleased fixes that were made specifically for this service pack. if you have either Windows XP original (with no Service Pack), you must install Service Pack 2 first, followed by Service Pack 3. Microsoft Office Compatibility Pack Service Pack 3 (SP3) provides the latest updates to Office Compatibility Pack. However, Service Pack 2 includes all of Service Pack 1a. You can't apply SP3 unless you have, at bare minimum, Windows XP SP1a. Microsoft stopped supporting the early releases of Windows XP.
*This next bit is going to get confusing.One notable exception is that, SP3 does include Network Access Protection (NAP)."
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.