How to Recover Disk Space After Uninstalling a Game?

I'm having a frustrating issue with my computer not recognizing that I've uninstalled an 80GB game. The files don't seem to exist anymore, but my computer insists I still need space for new installations. I discovered that the files may still be present in my D: drive, while my computer thinks they're on my C: drive. How can I force my computer to free up that disk space? I'm not very tech-savvy, so any detailed help is appreciated.