Wanting to make a source game anthology, how to not recompress shared files/libraries
 

Basically the title, in the games Counter Strike: Source, Portal, Half-Life 2 there are many shared files, as after all, they're all source games! What I would like to do is to have all of the *shared* files in one folder, and when a game is chosen to be installed it will decompress it's game specific files, and then also pull from the shared folder.

Now, if this was using cli unix tools it would be pretty straight forward, but I'm wanting to do this as a GUI windows installer, which I'm pretty unfamiliar with.
Any tips/advice?