Many apps can be installed into an alternate location (not into
Program Files), just choose the destination disk/directory during the installation. However, some are poorly tested and fail in strange ways if you install them anywhere else but in
Program Files. E.g. it lets you choose the destination directory, but path to one of the needed files is hard-coded to
Program files somewhere in the program code.
Moreover, some applications install huge caches and/or temporary files into your system disk no matter where you install the application itself (Adobe suite, many Microsoft programs, Google earth maps, etc.). So, they will still use up space on your OS disk even if you install them somewhere else.
So, a better long-term solution would be to increase the size of your system disk beyond 20GB. That is too small, won't be enough for OS + updates alone.