Contrary to popular belief, you don't need to go to a boxing gym and get pummeled by some goon to get a great boxing workout, you also don't need to buy a Tai-Bo video, if fact you don't need much of anything but the desire to become more physically fit.
Consider what you've seen from boxing training in the movies and on TV: running, skipping rope, push-ups, sit ups, heavy bag. You can leave out all the hokey stuff like pounding raw meat, chasing chickens and lifting your entire family in a horse carriage.
All you need is your body and a jump rope(you can get one for around three bucks, not much of an investment when you consider the cost of a gym membership).
Perform your exercises in rounds of three or four minutes. Run or skip rope for three minutes, then get down and do a set of push-ups, then take a one-minute "between round" break, then run or skip rope for three minutes more minutes followed by a one minute session of shadow boxing then another one minute rest period. Pay close attention to form when shadow boxing, use both arms equally and vary punches: jabs, crosses, hooks and uppercuts. Take care not to fully extend your arms on straight punches, this can cause injury to the elbow.
Continue for 8-10 rounds of running, alternating the exercise you perform after each three minute runs with push-ups, shadow boxing, bodyweight squats, sit-ups etc.. This should take around 30 minutes or so and, done with proper exertion, will give you a tremendous full body workout.
As always, consult your physician before undertaking any new forms of exercise and happy training.