Just about any of the workout routines out there now will work.
The key is you have to DO them. P90x 1 or 2, any version on the Insanity's (much harder if you're out of shape).
P90x is an amazing routine for the novice. It is extremely structured and keeps the average person engaged if they actually WANT to do it. 3 lifting days, requiring nothing more than a set of bands or a pair of dumbbells. 3 cardio days.
I've talked a number of friends into it, and the ones that actually stick to it (it's 1 1/4 hours max per day) have dropped 3% body fat and 15 pounds minimum. (15 pounds doesn't sound like much for 90 days but you are adding muscle in the process).