Autodesk warns that if you need to open Mechanical Desktop files in 2010, then do not uninstall the older version (2009 or prior).
From the Autodesk webpage:
Autodesk Mechanical Desktop 2009
Autodesk Mechanical Desktop has been discontinued and starting with the 2010 release is no longer included with any Autodesk Inventor product.
To open Mechanical Desktop files in Inventor 2010 make sure to leave the MDT 2009 installation on your computer.
If you are a previously licensed user of Mechanical Desktop software and have uninstalled Mechanical Desktop 2009 or are installing Inventor 2010 on a new computer then you will need to download new installation files that are compatible with Inventor 2010.
The new installer for AutoCAD Mechanical Desktop 2009 is provided by a self-contained installation file that you can download here. Or, previously licensed users can also request DVD media at no additional charge*.
*Free products are subject to the terms and conditions of the end-user license agreement that accompanies download of the software.
If you need to re-install MDT again, then you can download it from here:
Ensure you keep your MDT 2009 serial number too.