SQL Server 2000 DTS Designer components are required to edit DTS packages.

I've got this error when opening DTS package in Microsoft SQL Server Management Studio:
SQL Server 2000 DTS Designer components are required to edit DTS packages. Install the special Web download, "SQL Server 2000 DTS Designer Components" to use this feature. (Microsoft.SqlServer.DtsObjectExplorerUI)

Problem was fixed after changing order in Path variables (System Properties/Environment Variables/System variables):

C:\Program Files\Microsoft SQL Server\80\Tools\Binn\; should appear before the
C:\Program Files\Microsoft SQL Server\100\Tools\Binn\;


Incase of 64 bit machine:
C:\Program Files (x86)\Microsoft SQL Server\80\Tools\Binn\;

Please make sure to restart Microsoft SQL Server Management Studio after changing path.