I agree with wheelsup. You should be able to install a box copy of 1946 in a Windows 7 machine. You'll need to mess with the security settings a bit. Just follow the instructions at the Mission4Today link he mentioned. I also agree that you might have problems upgrading and modding the the Steam version.
You can downloadindividual mods or opt for a mod pack. If you're like me and not super confident messing with file structures in the game folder, you might try a mod pack, such as HSFX or UltraPack (UP). HSFX installs directly over the 4.11.1 patch; I'm not sure about UP. In addition to giving you gobs of new planes and maps for full mission building not in the official releases, you will be truly amazed what the mods do to the game's appearance and sound. It's almost like a brand new game.
Whatever you do, make sure you create a copy of your vanilla 4.11.1 game as a backup. You can mess things up with mods and it's a real pain having to go all the way back at the box game and work forward.