AS green granite says: if you've installed a new operating system (or reinstalled the existing one) then you also have to reinstall the software - from the original install disks, or from the "installer" you downloaded. The existing old copies on the PC prior to the new operating system won't work.
Well, some old stuff dating prior to 1993 or so may work, but that's not the way to plan.
Internal or external HD makes no difference. I prefer to keep the software on drive C and my "stuff" on the other drives.