>A client of mine has an accounting program on a Win2003 server.
>The accounting program uses MSDE (vers.2000).
>I've now the need to install SQL Server Express 2005 and the Management tools.
>Is it possible to install Sql2005 Express without conflicting with MSDE so as not to generate conflicts with the accounting software?

Yes. Make sure that SQL Server Express 2005 is installed as named instance with distinct instance name.