No, ANSYS Workbench is not installed separately. ANSYS Workbench is a flexible simulation platform that should come packaged with ANSYS 12. If you installed the program correctly, you should find the utility if you navigate to Start > All programs > ANSYS 12 > ANSYS Workbench. If you can't find it, remove the software from your computer (Control Panel > Uninstall a program) and install it again. If you still have issues, I suggest you contact the developers.