I'm also doing P90X classic version. It's a real kick in the pants, isn't it? Definitely not for beginners! Like BluePhire and pshsx1 said, just do as much as you can and do less reps or stop if you have to do so. Or, you can do a workout every other day instead of every day.
I first tried to start P90X back around Feb/Mar or so and really struggled the first few days. Then, I had to stop when I went to firefighter academy. I got my behind handed to me on a silver platter at academy; but, I came out of it more fit. Now that I've just started P90X again, I am doing so much better at it because my fitness level was higher going in. That's why I recommend complete beginners to try other ways of getting fit first or just going really easy on yourself.
That's me in the photo above. It's a "before" photo. It's not brilliant, but probably not too bad for a beginner, a 33 year old lady who sits at a desk 40 hrs/week!
Good luck to everyone!
.....Kelly
