Issue: Unable to open Lync 2010 client.
Error: Lync has both per-user and per-machine instances installed.
This is purely application issue. When you have Lync Attendee User-Level Client installed on your computer and installed Lync 2010 client on same computer above issue will start. Whenever you are opening client you will see above error because two application instance found on computer.
Resolution:Resolution is very simple process. Simply uninstall Lync Attendee User-Level Client from affected computer.
1. Open Control panel.
2. Double click on ‘Programs’ and ‘Uninstall a Program’
3. In list of application find ‘Lync Attendee client’ and say uninstall.
4. Once uninstall Lync Attendee client restart computer.
5. Now check and open Lync 2010 client and sign-in.
6. That’s all.