I already did something like that, but I want to jump into actual programming instead of using pre-created programs. I used to create mods for Warcraft III but I stopped, also learning how to build games from 0 not from 10% is more valuable, since you will know all the structure and it is more valuable in general!
I suppose it depends what you consider "value". If you want to get a job in the industry some day then you will likely be working with an existing engine, unless you get a job on a tools or engine team, in which case you wouldn't be working directly on "games" per se. (I'm generalizing, mind you)
If you want to do things at lower level then I would suggest learning Direct3D and/or OpenGL, as well as an audio library like DirectSound or FMod. Maybe a Physics library like Bullet, too, depending on what you're trying to do.
For simple 2D games it's not too hard to wire something together using a combination of low-level libraries, though it can get pretty complicated once you get into 3D stuff (read: I hope you like math )