The default install path for Steam is into the C:\ drive in a "Program Files" directory, which on all "modern" versions of Windows, is a protected directory, and Windows will restore most of the altered files back to their original state. If you're Steam install is in there, you'll have to use your Steam app to add a library folder for your Silent Hunter install that is not in that Program Files folder. On a different drive is best, if available. You can also move folders, and then tell Steam about the move, such as:
https://support.steampowered.com/kb_...hp?p_faqid=231
I moved my whole Steam install when I did that on my laptop. Thing is, it re-creates all the online info, right back in a C:\Program Files \Steam folder, and in the new folder, so it takes a while... Read the whole thing a couple-three times so that it's stamped in your brain for more than 2 seconds (applies to old fahrtz like me). I even printed it out for reference as I went along...
If you've got a 2nd hard drive though, the first method there, where they describe adding a Library folder is the quickest, most efficient way to go.