ANY activity you get in is better than none at all. It's a myth that the only effective exercise takes hours per day. It's true, ideally you want a well-rounded program that incorporates cardio-vascular exercise, strength training, flexibility, etc. but sometimes we have to make compromises.
A recent study showed that a group of women who walked briskly for 10 minutes, three times per day, actually lost more weight in a month than another group who walked for 30 minutes, once per day. In terms of calorie-burning, those little bursts do more for your resting metabolism than a single, longer session. I know from experience that they do wonders for your energy level, too. That sort of routine doesn't do much in terms of cardio-vascular health -- you need at least 20 minutes in your target heart range for that. But, you could do the little bursts on your busiest days, and work in some longer aerobic sessions on other days.
There are lots of good videos around that are brief workouts ... Leslie Sansone's Walk Away the Pounds videos are typically 20 to 40 minutes. Kathy Smith's Lift Weights to Lose Weight (both the original and the more recent vol. 2) are divided into 20-minute sessions. Tamilee Webb's Total Stretch for Beginners DVD has 3 15-minute routines as well as a longer one.
A web site on women's fitness that is often recommended here is
www.stumptuous.com/weights.html. The "rant" for this month addresses this issue EXACTLY, and I think it would be really useful info for you. The URL to the article is
http://www.stumptuous.com/rantdec2004.html .