Just to amplify Jared's response...
Most PC users in my experience will have great difficulty "shrinking" the OS that's embedded with other data in a larger-sized HDD so that it will "fit" into a much smaller SSD. Truth to tell it just never seems to work.
Now there are a few data-migration (disk-cloning) programs that purport to do just this but our experience with them has been quite negative. Rarely are they able to manage SOLELY migrating the OS from one disk to another disk.
As a general proposition we recommend using a data-migration program to move a system from one drive to another drive only when the TOTAL volume of data (OS + everything else) on the source disk is LESS than the TOTAL disk-space of the destination disk.
So our suggestion to users who are pondering whether or not to use a data-migration program given a situation similar to yours is not to do so. Rather, fresh-install the OS onto your 120 GB SSD and use your HDD for whatever programs & data you need.