Besides what razark mentions, installing the game in a "C:\Program Files..." folder, which is the game installer's "default..." setting, leads to issues with "modern" version of MS Windows, from Vista to Win10, if you mod. You say you have a disk version, so what you can do is copy the whole "Ubisoft..." folder structure, and then "paste" it into the C:\ root (or a 2nd hard drive), such that you end up with a "C:\ Ubisoft \ Silent Hunter..." structure, without the interference from Windows that the Program Files folders have. You could keep the current install in the C:\ Program Files folder if you want to - but do not un-install it, else you'll eliminate important info in the registry. You will have to create new shortcuts though to where you moved the game to, or just double-click on the SH4.exe file to run it.
Other issues come up when a person shells out to Windows from the game ("Oh! I need to send an email to my brother!"), and then comes back into the game. I've got a dual-monitor set-up with my computer, and I'm accidentally doing that all of the time, because I'm bad about hitting a 19" piece of monitor real estate with my mouse clicks (I'm very excitable when under attack with depth charges...
). Similar issues would be having the CO2 level continually rising, even though you've surfaced your boat and are breathing fresh "air". There are a few others that I can't think of right now... (such a memory). The easiest way to get rid of irritations of that sort is to Save your game ("Replay" is not necessary). Exit / Quit the game out to Windows, then re-start the game and load your saved game. All should be quiet now, or the oxygen restored, as the case may be...
