By updating the build number to 6003 , Microsoft enabled a new code integrity level that told the OS, "Trust SHA-2 signed content." It also prevented older, unpatched software from trying to install SHA-1 dependencies incorrectly.
Although Windows Vista and Windows Server 2008 share the same codebase, Microsoft officially only applied this change to the Server edition. However, some users have used Server 2008 updates to "unofficially" bring Vista to build 6003. windows server 2008 build 6003