This error may be seen on Windows Server 2008 R2 and Windows 7, when two Windows session users (that are NOT members of the Administrators group) attempt to start the Windows Photo Viewer.
According to Microsoft, "The issue occurs because the Windows Photo Viewer uses the Microsoft COM+ hosting process (Dllhost.exe). This process is configured to allow only one instance of itself to run on each computer."
Please review this Microsoft KB if you encounter the Photo Viewer simultaneous use problem.
Article ID: 288