Well I've just spent the last few hours trying to install Sophos Enterprise Console 1.0 on 3 Windows Server 2003 systems. One of the server installs worked just fine and the other two failed with the following message:
Error 3500. The custom action "InstallMSDE" failed with the following error: Fatal error during installation. 1603
A Google search using "sophos error 3500" yielded nothing useful. A search of "sophos error 1603" proved more valuable. It pointed me in the direction of looking in the MSDE install log (C:\Documents and Settings\Administrator\Local Settings\Temp\SophosMSDEInstaller.log in this case). The line of interest was:
InstallSQLAgentSecurity failed (SERVER02,LocalSystem,87).
A Google search for "InstallSQLAgentSecurity failed" then yielded "Backup Exec 9.x or 10.0 for Windows Servers fails to install ..." which detailed the exact problem, but with Veritas Backup Exec 9.x/10.x.
One of the links included in this document was "PRB: The MSDE Installation Does Not Succeed If the Installation Computer Is Running Terminal Services in Application Server Mode".
The problem with this was I wasn't meeting all the criteria - I was using VNC to remotely install the software after it failed first time. Long experience with software installation on a W2K3 system even when accessing the console session over RDP has resulted in VNC being installed on my W2K3 systems.
Further wading through the Google result set then gave me this: "Solution for the WSUS/WMSDE install error"
This accurately described my problem - MSDE failing with the same error. Not only that, but it explained why one of my servers installed Sophos Enterprise Console correctly - it was the one with the FSMO roles! I excitely followed the instructions in the Microsoft Knowledge Base article titled "How to view and transfer FSMO roles in Windows Server 2003" and yay! A successful installation.
Performed the FSMO transfer again on the third server, then went home for much needed food and sleep.
Must allocate some time for the next time I need to do this to see exactly which FSMO role needs transferring.