Hi:

I want to put a new GFCI combination switch and outlet into the
spot where there was just a light/fan switch before.

The instructions they sent along in the box are for a GFCI
dual-outlet unit only, which makes the very good, detailed
instructions not so useful.

The GFCI unit is this model:

http://doityourself.com/store/0431924.htm

Everything is labeled quite clearly, on the unit. On the bottom
it's got the usual pair of screws for "Line": it tells you to
wire the black on one side and white on the other.

On the top of the unit are two other screws for the "Load";
again, one screw for the white wire and one for the hot black.

There are a pair of black wires jutting out of the top of the
unit like antenna.

The box I want to install this in (in the bathroom) has the Line
cable leading in on the bottom. Then, from the top of the box, a
Load cable exits to provide power to the medicine cabinet lights
and an overhead exhaust fan. I want the switch to control those
lights and the fan.

There is an external ground wire that runs separately from where
it's screwed onto the box down into the woodwork where I assume
it hooks into the breaker panel or wherever good grounds go to
get hooked up.

Nothing is as simple as it seems. I wired the Line wires, white
and black, into the bottom screws. Then I screwed the Load
wires--those that run to the lights and fan--into the "Load"
screws on the top of the unit. Not knowing what the black wires
jutting out of the unit were supposed to do, I capped them
temporarily. Of course, they probably have something to do with
the switch, because now when I restored power from the breaker
box, the lights and fan are on all the time and the switch does
nothing. Obviously I didn't wire the switch in. At this point I
realized I needed to learn more about what I was doing before
continuing and so I'm writing here.

How do I wire this up so that the light/fan switch works
correctly?

Another question. I was pleased to see that the box was grounded,
since this is a somewhat older house. I assume since the box is
grounded, I don't have to run a (short) ground wire from the new
receptacle/switch to the box? The screws holding the
receptacle/switch into the box will provide sufficient contact
for ground?

Also, while there is a ground wire running from box to breaker
panel, I don't think there is one running from the box to the
lights and fan. I probably should install one, right? (As long as
I have the walls open and the new drywall not yet installed.)
There will be no receptacle on either lights or fan but those
should still be grounded, right?

Thanks for any advice.