Assuming you have already spoken with a doctor...
Yoga with a good teacher helps old injuries. Check out Beryl Bender Birch's book Power Yoga (she's got good written information about old injuries) and look for a good Iyengar or Astanga yoga studio in your neighborhood. Birch's approach (astanga) is a little intense and a real time-commitment but more athletic or yang. Iyengar is probably better if your injury is pretty severe, if you have less time, or if you are a particularly analytical person. It focuses a lot on learning proper alignment, you do very few techniques per class and hold them a long time.
This is just my opinion, but I think other styles of yoga are less effective when it comes to training a body to deal with an acute injury - they just focus on different things, like breath/prana, flexibility, relaxation. I would especially stay away from Bikram schools, because they're severely franchised and the tone is very competitive, very no-pain-no-gain, which could get you re-injured.
It is important to emphasize the good teacher part if you're working through an old injury. Your body compensates for old pains in countless ways that you are completely unaware of. Yoga can only re-teach your body to align itself and be comfortable and strong if you break those habits, and you will never be able to do that with a videotape or book. Be dilligent, make sure this person has been teaching for many years, make sure they know your situation and feel confident about helping you. If your injury is severe, make sure the instructor has a physical therapy (or similar) background. Remember that any knucklehead with a pair of leggings can teach a yoga class and do resesarch.
Whoa, that got long... blah dee blah