Plus 1 on what ala_xfire just posted.
There are a lot of knowledgeable people on this forum who can answer your questions about how the security system works. About the only contribution I can give you is my actual experience removing the siren.
This was physically hard on me. Bad back - I just can't bend over the fenders or crawl under cars the way I used to - my point being, if I can find and remove the siren , then you should have no problem.
For me, removing the windshield wiper arms was a real pain. I fiddled around with a cheap-o puller, but it couldn't get an even grip, and it just kept twisting off. I finally gave both "stalks" a shot of BreakAway and let them sit for a while. The puller still did not work so I used an old ball joint remover to pop the arms off.
Removing the screws to free up the cowl was no problem, but I never did get the cowl completely off the car. Too many tubes and wires for a complete removal, but I got it loose enough to just peek in, find the siren and the bolt holding it on, and remove the siren.
I diddled around with the electrical connection to the siren. I thought it might have a clip that I had to release - but it just pulls apart. Once you release the electrical connector and remove the bolt, the siren just lifts out.
I did not use a hacksaw to split the two halfs of the siren. I used a Dremel tool with a cut-off disc.

The tutorial (PDF) mentioned by ala_xfire (above) is really good. You should also use the search function and keep typing in things like "siren", "siren keeps going off", "bad siren", etc.
You can do this.

Best of luck.