93 Regal Problem Starting

I went to start my regal today and every time I turn the key the only thing that happens is the horn blows. What could possibly make this happen? I know the battery and starter are both good and I checked all the fuses. This hasn't happened before and we've been trying different things to start the car for over 4 hours now. Any suggestions?

Some kind of aftermarket alarm./theft deterrent system. Do the lites work, dash lites come on?

Found it... We had already removed two aftermarket alarms and thought we had gotten them all out - but there was a third. It's all set now. Thanks!