When I went to install Microsoft Visual Studio 2005 Service Pack 1 on Windows Server 2003 I had a lot of unexpected trouble.
Windows Update gave me this obscure error code:
Error Code: 0x64C
So, I did some googling and landed on this very helpful web page:
Installing Visual Studio 2005 SP1
The main key was to look into the Event Log (how often I forget) and see this error:
Product: Microsoft Visual Studio 2005 Premier Partner Edition - ENU -- Error 1718.File C:\WINDOWS\Installer\66b5f.msp did not pass the digital signature check. For more information about a possible resolution for this problem, see http://go.microsoft.com/fwlink/?LinkId=73863.
I'm not even sure what "Premier Partner Edition" is. It seems like I have that edition, plus the normal professional edition of VS 2005 installed. Anyway, that helpful URL points to KB925336. That knowledge base article points to a hotfix for Windows Server 2003, which is apparently required before installing large updates (and VS2K5 SP1 indeed appears to be large).
I did dig up my original installation media and did have that in my CD drive when I eventually successfully got the service pack to install, but I'm pretty confident it was the hotfix that got everything to work and I probably didn't need the CD.
It did take at least 20 minutes for the update to install (not including download time), so be patient.