Absolutely.

Yes.  Don't be in too much of a hurry though.  Try to preserve as much
muscle as you can.

Start here:

http://www.stumptuous.com/cms/index.php

http://www.exrx.net

Both sites offer a lot of good exercise (and at Krista's site, nutrition
and motivation) advice.

Amy

    No, one should lift weights to spare muscle
    while losing fat.

    If you haven't been lifting all during your
    weight loss, which you should have been,
    you should start now.  And don't be surprised
    if your weight loss slows because the lifting,
    along with a nice low-carb diet that supplies
    adequate protein, will slow or stop any muscle
    loss.

    Thank God (or Atkins) for low-carb diets which
    tend to spare muscle even in the absence of
    weight training.

    Yes.  That's another thing you should have been
    doing all along.

    Weights twice a week, and cardio three times
    a week is fine as far as frequency is concerned.
    What's even more important is intensity of effort.
    Start off with exercise that you're easily capable
    of completing.  Then, as your confidence in your
    body grows, don't be afraid to make the exercise
    challenging.  This applies to women just as much
    as men, and to cardio as well as resistance work.

    When it comes to weight training be sure to include
    the key multi-joint exercises (bench press, deadlift,
    squat, row and overhead press).  Use weights that
    make 8-12 repetitions challenging.  Don't overdo
    the volume (number of sets or exercises).  Volume
    is a poor replacement for intensity of effort.
    Train slowly; 2 seconds to lift the weight and 4
    seconds to lower it.  Train less but train harder.
    An effective weight workout might only have you under
    load (where you are actually supporting and moving the
    weight) for 3-6 minutes (ten six second repetitions
    requires one minute of time).  The weight is heavy,
    the work is hard, and the rest between sets is long.
    Including rest between sets, the entire workout may take
    30 minutes, but you were under load for 10-20 percent
    of the time.

    The above is a proven formula that applies to genetically
    typical people (the vast majority).  It's especially useful
    to those who are employing a low-carb diet.  In other words,
    those who don't have a lot of stored glucose to power them
    through higher volume workouts (which are, in reality, less
    effective anyway).

    I hope this helps.
--

Consult with a doctor before beginning any exercise program, especially
after being overweight and sedentary.
Please don't take the exercise advice of anonymous voices on the internet --  
albeit well-meaning --who know nothing about your age, physical condition or
possible health concerns.