I play SF4 on a Windows PC using the keyboard. How do I perform moves related to circle movements (ex:- quarter circle fwd, half circle fwd) using the keyboard keys?
Okay, got it now.
This guide explains how to do perform these moves using the arrow keys.
An even better guide can be found here.
I've managed to figure out all the moves of ken (after properly mapping the controls) and it turned out to be the same (almost at-least) for many others. It wasn't really easy figuring them out. Make sure you have the movement controls as wasd, uio as punches, jkl as kicks, y as triple punch and h as triple kick(or whatever its called). The rest where ever you find convenient. The ultra combo for Ryu is sd sd and y. Check how the movement is in the command list and cross check for a similar movement in the below mentioned characters. Here's a list of players having a similar controls(i.e.combos) scheme - Sagat,Ken,Ryu,Sakura,Dan,Abel,Viper & Dhalsim.
Note: some characters may have a slightly different movement for certain combos. Thanks to that, I've managed to finish the game as Sagat,Ryu,Ken,Dan and Sakura in medium difficulty and yes, I am aware of every move of those characters.
Edit: You can pull of El Fuerte's ultra combo as well. Also, for shoryuken and similar moves in other characters - s(hold) tap d twice and press any punch/kick move (depending on the character).