One potential pitfall you may encounter CombatFlapjack, is the default install path for the Steam version of the game, which defaults to "C:\Program Files (x86) \Steam \SteamApps \common \Silent Hunter...." Windows will not allow you to 'mod' that install. If this is your only Steam game, just re-install, but put it in a folder of its own that you make, such as "C:\Games \Steam \" etc. If you have a 2nd hard drive or partition on your computer, you can create another Steam Library folder that is outside of the normal Steam folder. The Steam Support site has a thread
Moving a Steam Installation and Games.