Once you identify the file being held open use Process Explorer, a Microsoft diagnostic tool to find which process has that file open.
Use the online site to execute direct from the web
or a web page with it http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Either run it directly or save it and run it from your hard drive. You don't need to be an admin.
Make sure View Menu - Lower Pane View is set to Handles.
Click Find menu then Find Handle or Dll. To see which process has a file open type part of the filename and scroll to the File category.