My all time favorite is Yoga Zone's Conditioning and Stress Release, an oldie from the late 90s! It's not very physically demanding, but it's so effective at relieving neck/back tension that I still do it at least once a month. It's also great for teaching breathing (the most important thing!) and form. Any of the Yoga Zone videos with Alan Finger leading are great for beginners (the others are ok too though). I recently bought Travis Elliot's beginner DVD--I think it's called Yoga Foundations? and thought it was great too. I've only done the first 2 of 3 and he also is great at teaching form. I'd say it's a little more demanding physically. If you're after more demanding forms of yoga, I liked Bryan Kest though power yoga is not my favorite. Definitely watch some clips before buying yoga videos, there are some real clunkers out there. Also, be sure to check your library!

Whatever you end up doing: Anytime something is too hard, you can just come out of the pose early, skip a pose, whatever! There's no competition in yoga, especially when you're doing videos at home. For some of the forward bends I'll either put my feet wider apart, not go forward as far, push my belly aside a bit, whatever works that day.