Also not Cathe.
I've just started back to working out in the past few months. I did not buy any of the beginner DVD's because we're saving money and I tend to like the more complex choreography. I also like having DVD's that I can 'grow' with so that I can use them for a long time.
I actually had really good luck starting with my IMAX workouts (I have IMAX 2 & 3 and LowMax) and modifying quite a bit. I did the premixes so that I was doing only the first or second half of the workout, and I modified during the blasts so that it wasn't too taxing. You could even use the blast periods to rest at first. Anyways, I thought the IMAX workouts were good to start with because the short step segments made it easy for me to get back into the swing of things.
I also did shorter Premixes for many other workouts: Cardio & Weights, KickMax, Pyramids etc, etc. Just dont' be afraid to modify the workout to suit you - don't jump on the high impact parts at first, use really light weights and work your way up, skip a few reps of exercises if it's too much, etc, etc. I particularly like LowMax and KickMax because it's easy to get rid of most of the high impact aspects of those workouts, so they were easier on my knees.
I also started out by working out every other day so that I had plenty of rest days in between everything, and I think that helped a lot. Even taking it easy, it's a big shock to go from not working out at all to working out six days a week.
P.S. Also, although I've gotten a really good start at working out and have become much more fit in the past few weeks, I haven't lost any weight yet because I haven't altered my diet. Just wanted to remind you (and myself) that diet's a really, really important part of the weight loss equation.