The driver for SQL Server 2005 is called the SQL Server Native Client. It is installed by default on Windows Vista and Windows 2008, but nothing older. You can install the SQL Native client on your workstations from the SQL 2005 DVD/CDs. There’s an MSI which you can push out via group policy.
If you don’t need to use any of the new features of the new driver (such as Mirroring auto fail over) then you can continue to use the SQL 2000 driver without an issue.