How do you fix an antitheft alarm that comes on for no apparent reason?

My 1998 Altima has been doing the very same thing for the past year. My temporary solution was to pull the fuse to the security alarm. That got rid of the annoying spontaneous alarms; however, when friends/family borrowed the car and tried to start it, they had no idea what was wrong. Unfortunately one of these instances forced me to bring my car in to a Nissan dealer. They said I needed a new "smart entry system". I let them change it. $650 later, the car is doing the exact same thing. I am going to pull the fuse again. I realize this isn't the solution you're looking for, however, if you do bring it in and your dealer suggests a new smart entry system, tell them to try again... Good luck..