Hi Bernie,
Not sure if you need to have SQL 2000 MSDE installed or not, but I'd go with only having SQL Express 2005 if you can.
However, it seems possible to have them co-existing on the same server. Read up here:
http://dbaspot.com/forums/sqlserver-tools/428397-msde-2000-sql-server-express-2005-same-pc.html
As long as the default instance names are different, it appears possible, but I would suggest trying this in a test environment before doing this live.
Laters!