NX Viewer should be in your Windows start menu under Start /All Programs/Siemens NX 10.0/NX Viewer
This pertains to NX 10 & 11, and should be the same with older versions.
@akrenzer is right, after you have installed NX you will find the option, in your Windows Start menu as explained.
About the installation itself. You can just download the full NX install (there is no separate NX viewer installer). Run the installer, and select the option you want to install from the installation menu.
When installed, you will find the NX viewer shortcut in the windows Menu.