If you take the eliptical marquee tool (click the small rectangle on the rectangular one, top left) you can drag your circle or ellips.

shift makes it circular
alt makes it start from where you start
the spacebar lets you position it.

this creates a selection which you can feather (toolbar) or/and anti alias

copy to clipboard
activate other file
paste from clipboard.

this creates a new layer
the move tool lets you position it.

if needed you can stroke (edit>stroke)

another way would be to use paths, but that's more techie, and if you have problems with this, the method I described it the simplest one.

oh, and if you have repositioned layers, it's a good idea to select all (Ctrl-A) , invert selection and then delete. this gets rid of pixels hanging in thin air that you don't see.

and if you're looking for tutorials: for $25 you get a one month membership at lynda.com . Choose Photoshop CS2 and watch Deke McLelland on Channels and Masks. He's a bit of an ego (to say theleast) but his knowledge is jawdropping. And he's a good teacher. (he's funny)