Car wouldn't start, Aftermarket alarm? Options

[BlackSTX]

Don't know where this should go exactly. My car wouldn't start the other day. I even replaced the battery and tried several times and it still woudn't work. So I had it towed to the dealer and sure enough it starts right up for them. They told me the only thing they can think of is that it's an aftermarket alarm system that's installed may be interfering somehow. I didn't even know it had a system since I bought it used. Anyone know if this could be right? When it wouldn't start, it would turn on, meaning the battery was good but it just wouldn't start.

[Tweek]

That sounds right. I had my remote go bad on my car and I could'nt get my car to move until I got the new remote. (I forgot my manual override code) I ended up drunk in the middle of nowhere after a party with the alarm going off. ( I had to open the door manually which set it off cause the last thing the remote did before it died was start the car, but it did not unlock, so I had to get in to turn the car back off. It was a long night)