Wow! your game sounds like it might be as old as my original version, which I purchased in March of 2007. What you need to do is to
download the v1.4 Patch which should allow you to not need the disk to run the game. Don't do what I did years ago though, and put the disk away for safe-keeping, and then forget where that "safe" place is...
Also of interest to you might be a v1.5 Uboat Missions Add-on, which will cost more money, but will update the game to v1.5 and get you the Uboats. I have not seen a disk available for some time now, but the download is still out there on the internet. Or, you might find the full "Gold Edition" available for a good price.
