I went to install it in a Windows Server 2003 virtual machine on my SBS2003 server. I installed a new instance of SQL Server 2000 and then upgraded it to SP4. The MOM2005 installer then failed the SQL prerequisite check - no SQL instances found! I then tried installing it on my SBS2003 box. It found only one SQL instance - my WSUS installation, but didn't think it was SP3 or later.
I then proceeded to find MSKB 902803, which gave me a modified msiexec install, but appeared to be targeted to MOM2005 and not the Workgroup Edition. I then managed to find a conversation in an Ars Technica forum. The process in summary is:
Load the registry editor and browse to:
Change the CSDVersion key from 8.00.2039 to 8.00.761
Instances can be found at:
HKLM\SOFTWARE\Microsoft\Microsoft SQL Server\
Run MOM 2005 Workgroup Edition setup
Change the value back when you are done.
This was the technique I used to successfully install MOM 2005 Workgroup Edition.