Here is some background that led to the decision to take this dependency on Windows Installer (MSI) 3.1v2 in MOM 2005 Service Pack 1. I completely understand how this dependency may complicate or delay your deployment of MOM 2005 SP1 particularly in highly locked down environments.
This Windows Installer 3.1v2 update (version 3.1.4000.2435) was released as a High Priority update on 5/12/2005. Installing the Windows Installer update generally requires a reboot following installation. If you have not installed this, it is useful to consider bundling the Windows Installer update with recent security updates that have been released.
Finally, note that it is fine to leave agents at RTM version after the server is updated to SP1. This is a fully supported scenario and allows you to get the MOM servers to SP1 first and then update agents as managed servers get Windows Update 3.1v2.
Feel free to leave comments - I appreciate your feedback.
Thanks!Dale Koetke (Lead Program Manager)