Pelican Parts Forums - Scary problem with starting, PLEASE HELP

I desperately need feedback on this one....what happened tonight with my '89 325i is definitely not normal and got me worried...

I was waiting for my girlfriend to come out of her building, and I shut the car off while I was waiting because she was taking too long. The headlights were on when I shut the car off, and I turned the key back just enough to kill the engine but leave the parking lights on so my girlfriend could see the car when she came out. When she came out into the parking lot looking for the car, I beeped the horn because she didn't notice my car right away. Anyway, when I went to start the car again, nothing happened. Every electrical accessory (lights, windows, blower motor) worked fine, but when I turned the key I didn't even get a click. It was just like I was trying to start the car in gear (it's an automatic model). However, the tranny was in PARK and no matter how much I messed with the shifter, I couldn't get the car to start. I checked fuses and all looked good. So, I went inside to make a phone call to my father who used to have an '85 318i, and see if he knew anything about BMW-particular starting trouble. When I came back out to the car, I tried starting it again--just to see what'd happen---and the car started up, but turned over sluggishly.
I drove the car straight home and shut it off; then started it back up again. It wouldn't start at first, but I shut the headlights off and tried it and it started up. I repeated this 3 times successfully. Then I repeated what I did in the parking lot where the car originally died---and guess what---the car died again. No matter what, it WOULD NOT even attempt to start as long as the headlights or parking lights were on. It never had a problem doing that prior to tonight. I decided that it could be the battery after consulting my Bentley Manual, swapped the battery out of my Camaro and into the BMW, and it fired right up just fine. It now starts with the lights on and everything. Just one thing that left me freaked out about the whole affair: after I put the Camaro's battery in the car, and tried starting the car after turning the headlights on first and beeping the horn before turning the key, the car turned over very slowly and made a sick noise and the lights on the instrument panel went dim with it---like there was a huge drain. I repeated this a couple more times to see what would happen, but it's acting normal now. The Camaro's battery is in very good shape--I know this for certain.
Please let me know what you guys think it could be; I don't want to take any chances and get stranded someplace. Does it sound like it was just the bad battery? It's been very wet today, is it possible that moisture in the wrong electrical spot would cause this? Does it sound like something more serious, like a short? The battery does not go dead on its own, so it's not like anything is draining the charge out of it. Anyway, let me know what you guys think. If you know anyone else who'd know, please ask them. I really need your help on this one. Thanks guys, sorry for the length here.