Lync has both per-user and per-machine instances installed. Click OK to remove the per-user installation
Issue: Unable to Open Lync 2010 Client.
Error message: Lync has both per-user and per-machine instances installed. Click OK to remove the per-user installation.
This is purely client side issue, when multiple instances installed on your computer and you are trying to open Lync client, you will see above error message.
Even error message also state same message that, In order to resolve issue, you must remove addition client instances.
Resolution / Workaround:
Resolution is very simple, just need to uninstall the Lync 2010 attendee Client instance. You may follow below steps:-
1. If you have Lync 2010 attendee client installed on your computer using User Level is installed then, uninstall the Lync 2010 Attendee Client - User Level Install. If not then you need to install and remove the same instance.
2. Install the ‘Lync 2010 Attendee Client - user Level Install’
Download and install Lync 2010 Attendee Client - User Level Install: http://www.microsoft.com/en-us/download/details.aspx?id=15755.
Note: Microsoft Lync 2010 Attendee is a conferencing client that allows users without Microsoft Lync 2010 installed, to participate in online meetings. If you are not an administrator on the computer but an administrator has allowed you to perform installations, you can install this application.
3. Later again you need to uninstall the installed Lync attendee client i.e. ‘Lync 2010 Attendee Client - User Level Install’ from Control Panel.
4. Now you can install the Lync 2010 x86 or 64 bit client per requirement.
5. You will be able to open Lync 2010 client without issue.