To add on to what Barry said, there is no way to take a backup of a 2005 database and restore it on 2000, nor is it possible to detach a SQL 2005 database and attach it to SQL 2000. The file structure is different, the system tables are different.

And before anyone suggests compatibility level, that just affects the T-SQL parser and the query processor, controlling how commands are interpreted. It does not affect the structure of the database file in an y way.