When I try to open Handheld Contact on the PC, I get a message 'There is already a copy of the application 'Handheld Contact' running. Please close that application before starting a new one'

In order to resolve this, you will need to first determine if Handheld Contact is already running on another PC.  If this is a server, then its possible that another user is logged in and has Handheld Contact open.  Otherwise, open the Task Manager.  Go to Processes and select the option to show processes from all users.  Then search for ‘MUPlugIn.exe’.  You can select it and choose End Process.  Once that is done, try reopening Handheld Contact and it should begin to startup.

This error message is also tied to the Message Queuing services not being installed.  You may also have this error appear.  If so, then please see: https://kb.handheldcontact.com/index.php?View=entry&EntryID=138.