Connecting SQL Server 2008 R2 to SQL Server 2000
Running SQL Server Management Studio on SQL Server 2000 is not really as supported option. Your best bet is to either upgrade the 2000 instance or to use SQLCMD (which should still work) to manage the instance from the command line.

You might be able to connect to a SQL Server 2000 from management studio 2005 (if you can find the install media).