Microsoft has made Windows Vista and Windows Server 2008 Service Pack 2 (Build 6.0.6002.18005) available for public download. You can get x86 stand-alone installer (348.3 MB) from here and the x64 SP2 (577.4 MB) from here. Download was pretty fast (under 5 minutes) for both the installers. The x86 Service Pack 2 installation on my modest Vista SP1 VM rebooted once and took around 40 minutes for the entire installation process to complete.
