This seemed to be way harder to find than it should be. There’s a bunch of threads about manual installation of the codec all over the place, including a few on Doom9, but none of them seemed really clear about how to just get the VC-1 codec installed on Windows Server 2003 for playing back/decoding VC-1 content.
After some enterprising Google searches (like “vc-1 codec for windows server 2003”), I finally found this KB article which includes links to x86 and x64 versions of the codec as “hotfixes”. Here’s the links and their md5sums as I downloaded them:
de8a19b41b0d4dbe5e4c243f5d8c273d windowsmedia-kb942423-x86-intl.exe
0af30dbc5f7ec947f5e1f618722c57c1 windowsmedia-kb942423-x64-intl.exe