Windows Server 2003 Service Pack 1 (SP1) delivers on the promise of Microsoft's Trustworthy Computing initiative and illustrates Microsoft’s commitment to creating software products with enhanced security, increased reliability, and simplified administration. SP1 builds upon the impressive track record of the award winning Windows Server 2003 operating system by providing refinements and supplying cumulative system patches.
More info:
http://www.microsoft.com/windowsserver2003/downloads/servicepacks/sp1/overview.mspx
Download link:
http://www.microsoft.com/downloads/details.aspx?familyid=22CFC239-337C-4D81-8354-72593B1C1F43&displaylang=en