Cardio workout without use of knees/no access to gym?

[uriel]

I've decided to get into shape and have taken up running. Perhaps unsurprisingly, this has hit my knees pretty hard and last time I was out, all was going well until when I landed one stride, something happened and I had a sharp pain in the knee. After that each step walking hurt and it's still not better a couple of days later. I've had this happen before (not when exercising, just regular use) and it'll go after a week or so.

Trouble is, I was really starting to enjoy my exercise routine, and was making really good progress (9 lbs lost in the first week). I'm not a member of a gym and can't afford to change that at the minute. I have free weights and a rowing machine. I can use the free weights, but that's really not the kind of exercise I think I need. There is the rowing machine and I've yet to give it a shot, but I don't think it'll be good for my knee.

I'm struggling to think of someway to work out that will get me that heart racing, panting for breath exercise I get from running. Not really into swimming.