I also recommend that you look at doing some strength training that builds up strength and stability in your core muscles and also in your legs. This will help reduce your risk of injury when you do start running. I have some exercises like this listed in a blog from about a month ago, if you're interested. You can also look at options like Pilates classes (really great for building core strength). I would recommend that you stay away from using the weight machines to build leg strength if you've had knee trouble in the past.
I also agree with others who suggested that you work up to being able to walk consistently at a brisk pace for 30 minutes before starting a C25K-type program. And when you do start that program, don't feel like you MUST progress at the rate stated in the program. It's more of a ceiling (the maximum you should do per week, and the maximum progression from week to week), rather than a minimum. It's better to build up slowly, and to let your body adjust over the course of a couple of weeks to an increase in mileage, time or running duration. This reduces the risk of injury and is likely to increase the likelihood that you'll enjoy it over the long term.
Even just adding a short running interval to a walking workout can make a significant improvement in your endurance, fitness and calorie-burning. Don't feel like you have to progress to running for 30 minutes straight in order to be a "runner". (I've done 11 races this year and am about to do my 2nd half marathon, and I almost never run for more than 4 minutes at a time.) I did my first HM with a run/walk interval of 1 minutes / 2 minutes (i.e., walked for twice as long as I ran) and it felt great.
Finally, before you start a walk or walk-run program, make sure you get fitted for a good pair of running shoes, and get them at a running store (not just a big chain sports store) where they will watch you run or walk and analyze your gait before suggesting a shoe.
| current weight: 4.0 over