For Rift specifically the installation itself is portable, simply cut and paste your Rift directory to where ever you want it on the spinning drive, and make a short cut to riftpatchlive.exe and move that to your desktop, rename the shortcut to Rift.
If it is not currently installed and you are downloading the installer from the website it will ask you where you want the game installed during the installation process. Just make sure that is pointing to a directory on your spinning disk. If you install it in the wrong location, remember... it's portable, just follow the instructions above.