Adobe After Effects CS3 Question

Basically, I have a small issue, when im editing in after effects CS3 on an intel imac and im dragging the slider of a particular effect's controls, say im trying to change the tolerance of the color key effect on an image after keying out the colour red, it only shows me the effect of the movement of the slider which i have done with the mouse once i have released the mouse button, when in fact i want it to show me the outcome AS i move the slider so i see the outcome as im moving the slider with my mouse as opposed to once ive let go of the mouse button,

I have After effects cs3 on a g5 power pc at work and it doesn't do the same thing, but I cant figure out what setting it is that i need to change,
Does anybody know what it is i need to change?

Thank you in advance

Davey