Codes

Choose Same Character Twice in Group Mode
Go to group mode. Where it has the 2 modes to choose from, on Controller 2, press Down,Z,Up,X,A,Y,B,C. You should here Chun Li say her voice as if she was doing her whirlwind kick. It can also be done by using controller 2 and pressing Down,C,Up,A,A,B,B,C.
Five Star Mode in Champion mode
When the Capcom logo fades, the building will show up with the Street Fighter II logo. When the screen stops going up and you can see the logo, on a 6 button controller, press Down, Z, Up, X, A, Y, B, C. If right, you should here a short Zangief sound. It can also be done on a 3 button controller by pressing Down, C, Up, A, A, B, B, C
Only Special Moves
Press Down+Z, Up+X, A, Y, B,C. Now you can only use special moves when fighting.

Secrets

Alternate Character Colors
When picking a character, press Start to select him/her. He/she will have an alternate colored uniform/shorts/etc.
Change Button Configuration in Arcade Mode
After choosing your fighter, when the plane flies to the next location, press and hold Start to bring up the controls. You can then change your controls as you see fit.
Hidden Ending
Complete the game with any character with the difficulty level on 4 or higher.