Do you mean "Lock transparent pixels" ?

No, it's definately called Preserve Transparency. I've seen pictures of it on other versions of Photoshop, and it's there when I go to Edit > Fill, but for some reason I can't find it on the layers pallette.

from Photoshop CS help:
"Lock Transparency to confine editing to the opaque portions of the layer. This option is equivalent to the Preserve Transparency option in earlier versions of Photoshop."

Aaah! Thankyou so much! I really appreciate your help. And sorry about posting the message six times, I'm new to this :P