I think you're looking for the "locked transparency" option... its part of the Layers Palette and is just an ICON rather than the "checkbox" in eariler versions of PS...

:)
retrocity

as for Alpha Channels, do a search on the subject in this forum, i think "hoof" just recently answered another member regarding this a little while ago. (and a very good explaination if i remember right...) you could also hit the F1 key while in Photoshop for the HELP files and go through that. I know manuals can be dull and boring to go through but they are the first line of attack when wanting to learn someting... if you have any problem trying to figure out what the manuals are saying, just ask, we'll try to decifer it ;)

oh, and welcome to the forum

:)
retrocity