Disable Factory Alarm?

Here's the problem... I bought this car used from my mother in law after her father passed away (it was his car). He lost the remotes YEARS ago. I've always just used the key to unlock the car, but I always end up with one problem...

The car was just serviced (had its serpentine belt replaced) and I assume they unhooked the cars battery. Now I'm having a problem.

I lock the car using the power locks in the door, and when I come back to the car and attempt to unlock it with my key, upon opening the door the alarm goes off. I need to disable this. I don't have the remotes. How do I go about accomplishing this?

what yr and model

Oops... sorry... 1998 Pontiac Bonneville SE (non-turbo)