One more question:
Or does it start to turn over but then slows down and stops?

Ok - are your dash lights coming on?

You checked the fuses but did you check the fuseable links as well?
They are under the hood in the fuse compartment.

If I told you to find your starter, would you know where to locate it?
Are you comfortable doing minor repairs to your car. I don't want to
suggest anything that could be dangerous to you or your car.

The following things are simple:
If you know where to find your stater, open the hood and have someone
try and start the car while you listen for a 'click' coming from that
general direction.
If you hear a click, it is most likely not your antitheft system or
ignition switch but most likely your starter or a wire going to your
starter.
In that case, get a piece of wood (like a two foot two by four). Place
one end of it on the starter. Whack the other end of the piece of wood
with a hammer. Don't hit it too hard - you don't want to ding the
starter, just mechanically shock it. There's a pretty good chance that
it starts if it is the starter. At least you'll be able to drive it to
a repair place this way.
Not knowing how comfortable you are being under the hood, I am
reluctant to have you try this: you could check the fat connection from
the battery to the starter. Clean both sides well and secure them well.

If you don't hear a click, it might still be your starter but will take
some probing around with a volt meter -- again, are you comfortable
doing that? In that case, it could be your starter, theft system,
ignition switch.

Nissantech is most likely right -- my money is on the starter as well.

Have you tried starting it with the transmission in "neutral"?  Sometimes
my 93 Altima won't start in "park," only in neutral.

I agree that there's a good chance that it is the starter, but based on
what she had indicated I don't think you can be sure yet, can you? The
battery wire being totally off would do what she has mentioned thus
far.