How to play pc games with my xbox 360 controller ?

I want to play gta SA on my pc with my xbox controller. But the problem is when I go to the configuration settings it won't allow me to set all of the buttons on my controller. For example: to walk I want to set the analog buttons (or whatever you call them you know the directional buttons with the rubber on them) but it won't allow me to because the only buttons it will allow me to set are the are the A B Y X SELECT START RB LB and the directional buttons when you press them down. It won't let me use the right analog stick either because I wanted to use that one to change direction. Any idea on how I can set up the controls to fit my xbox controller because I just can't play with the keyboard. That's just not the way to play a gta game