NOTE: Just press Ctrl and Tab together during a race. a picture puzzle will pop up on screen. This is the bullfrog logo jumbled up. Solve the puzzle by moving the pieces around with the arrow keys. After the puzzle is solved press Ctrl and Tab together and the puzzle will dissapear. Then you can try the cheats (Alt-F1 up to Alt-F6 , i think). this worked for me. only disadvantage is that while you solve the puzzle the race goes on without pausing or stopping and you have no control over the car until Ctrl+Tab is pressed. To regain control of the game at anytime press Ctrl+Tab, But the cheats will activate only if the puzzle is solved.
see ya,
Sirraj.