Lol dancraft it's not that hard; all you have to do is find a copy of the jar from 1.7.3, go into your minecraft folder, then into bin, and replace the minecraft.jar in the folder with the one you downloaded. It shouldn't harm anything, unless you screw it up in anyway, which is hard to do, considering how easy it is to do.
I'd post my .jar but it's riddled with assorted mods, some of which I'm sure you don't want.