Mount mdf file to sql server desktop edition
I suspect this is not a SQL Server 2000 database you are attempting to attach to SQL Server 2000. That appears to be a common cause of the error as indicated here also.

Can you attach this to SQL Server 2005 or 2008 Express and see what error message you get? Do you have access to the underlying database that your files are from? 

I imagine if you attach this to SQL Server 2005 or 2008 you will be able to attach it unless there are other impediments to attach going on that that approach will reveal.