I will second Mel's advice. Running is a pretty nice activity IMHO, but starting with it at your current weight may very well lead to more injury than results. I started running (although I stopped for more than one year in between) when I was about 138-140 lbs, and the very first time, I only managed to run something like 10 minutes, with a heart rate shooting to the 200-210 range (ick!)--and that was after months of doing elliptical and bike cardio at least 3x a week. So allright, I may just be crappy at it
, but nonetheless, it IS very taxing, all the more so on the joints.
(Uh, reading this again: I hope this will NOT discourage anyone from trying to run, now or later!
Really, it's the first or first two times that are really painful afterwards; the body is a wonderful machine that can adapt very fast.)
Anyway, the elliptical machine sounds like a pretty good suggestion. Maybe alternate with the stationary bike, too? You'll want to put the less pressure possible on your joints to start with. Maybe you can also up your walking rate (power-walking, or whatever it is called in English), to complement other cardio exercises at the gym?
And if one day you are ready to run (I say 'if' just in case you'd suffer from joint ailments or something similar that would prevent you from doing so--I do'nt know your medical background), just make sure you have good running shoes first. Bad ones can really ruin your knees and hips.
In any case--I hope you get where you want to!