If you view the history of an item that was renamed, you will see an entry like this in the history dialog:

123    rename    username    06/02/2008 09:11:55 AM    Comment

Then you try to open the changeset details to know to old name, but it only shows the new name:

NewName.cs    rename    $/Proj/ParentFolder

If you want to see the old name, you can use the command-line to view detailed history of an item:

tf hist /format:detailed /i $/Proj/ParentFolder/NewName.cs

The history entry preceding changeset 123 will have the old name:

 

-------------------------------------------------------------------------------
Changeset: 120
User: DOMAIN\userName
Date: 05/01/2008 04:10:05 PM

Comment:
  Change class name

Items:
  edit $/Proj/ParentFolder/OldName.cs

Check-in Notes:
  ....etc.