If a game require Steam, there is no legal way of launching it without Steam running (I don't know about Reactor, but it should be the same thing).
On the other hand, there are way of reducing the memory and CPU consumption of Steam:
- Make sure to pause any downloads before starting your game (Steam should do it automatically, but better safe than sorry);
- Disable the Steam overlay (for just a game, right click on the name of the game, properties, uncheck the Steam community box. You can do it for all games in the option menu).
With this, you can keep Steam's memory consumption under 50MB of private memory, and cpu usage low enough to not worry on a dual core CPU.