You can now download Windows XP SP3 from http://technet.microsoft.com/en-us/windowsxp/0a5b9b10-17e3-40d9-8d3c-0077c953a761.aspx.  The last service pack released for XP was 3 years ago and since then Microsoft has released hundreds of hot fixes.  SP3 wraps all of those fixes together and introduces a couple new features that will improve Windows Server 2008 compatibility.  SP3 will be available on Windows Update shortly but in the meantime, you can download it directly from TechNet.

Windows® XP Service Pack 3 (SP3) includes all previously released updates for the operating system. This update also includes a small number of new functionalities, which do not significantly change customers’ experience with the operating system. This white paper summarizes what is new in Windows XP SP3.  http://www.microsoft.com/downloads/details.aspx?FamilyID=68c48dad-bc34-40be-8d85-6bb4f56f5110&displaylang=en