From the documentation on the (I believe depreciated, but used in SQL Server 2000) sp_attach_db stored proc:
A database created by a more recent version of SQL Server cannot be attached in earlier versions.
I think you'll have to script the schema and data of your database.
Pinal Dave has a good tutorial on how to do this here - just remember to make sure you choose the following settings:
Under General:
- Script for server version: SQL Server 2000
Under Table/View options:
- Script data: True
Remember to double check the settings for any others you might be interested in, such as foreign keys, triggers etc.