You can use Scid as a GUI, you can download it for free. And then use any UCI engine you want, I recommend stockfish (also free). Scid also comes with Toga which is pretty strong too.
In Scid you have an option to put the engine analysis on low priority, so it doesn't take much cpu. Keep in mind that while you are not running the engine your computer won't slow down just because you installed the engine.
I have no experiences with chess engines but I would like to have in order to improve skills. First at all, I have Windows7 (or Vista) interface. I have considered to get Rybka (they say it is the best & free) but I saw that I needed to download something before downloading Rybka (something like GUI). I don't know what it is and I don't know do I need to dload anything else. And the worst, I fear that all this would change my regular computer.
Since I would like to keep my computer as it is at the moment, I wonder if something changes after having Rybka in my computer. I don't know what to do. Simply I just avoid dloading Rybka but it would be really cool to have a chess engine.
Helps! I would like to know the effects of chess engines to weekday computer actions!