There is a known issue (SPR MPGY7J6SJQ) that discusses problems with the client hanging after the install when “UseBasicNotes=1” is set in a multi-user environment.
For whatever reason, after the install completes, it leaves a Notes task running in memory–which prevents the client from launching again.
You can use Windows to “end task” the offending task or have the user log off & back on to kill it if this is the problem.