According to your system specifications you have a 120GB SSD (boot) and a 500GB HDD installed.
You should try to move your movie and picture libraries to the 500GB hard drive, which should clear up space if it's becoming a problem.
use a program such as SpaceSniffer
to see what is using up space on your hard drive, and what you can move.
There are two reasons why you sholdn't install Windows on your WD passport:
a) there will be a drastic performance loss. You will be going to a high speed solid state to a hard drive on (I assume) usb. it will go from running very quickly to being unusable.
b) Windows does not
like to run from external drives. Aside from being very slow, if your drive gets disconnected accidently at the wrong time your whole system can be screwed.