MOM 2005 Workgroup (or full edition) will not run on SQL 2005 Express, because SQL Express does not have the SQL Agent (needed for grooming and optimization jobs): http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnsse/html/sseoverview.asp.

MOM documentation is not correct about this:http://www.microsoft.com/mom/evaluation/sysreqs/default.mspx. This should be corrected shortly.

MOM 2005 Workgroup (but not full edition) will run on SQL 2000 Desktop Edition (MSDE).