Windows Vista is getting its first Service Pack while Windows XP is getting its last.
The release candidate of Windows XP Service Pack 3 is available for download. Service Pack 3 is more of a rollup of hotfixes and updates rather than a feature/experience enhancement. In short, it contains things that make Windows XP more secure, more reliable and faster. The user will not see noticeable change in the experience in general.
The Windows Vista Service Pack 1 Release Candidate is also available for download. There are large number of improvements for performance, security, reliability, interoperability, hardware compatibility, and deployment. There also some improvements for power consumption, desktop administration and is better geared for Windows Server 2008, the next version of Microsoft’s server OS.
Please note that Release Candidates are not the fully released versions and you will not receive support for it. Release candidates are for testing purposes. Please wait for the final release of the service pack before deploying to production systems.