2007 C70 conv Alarm goes off. Key wont work

[ros4260]

I have a C70 convertible that in the past couple weeks alarm has sounded on car(no flashing lights or red light on dash) cycles for about 6-7 times. Cant stop alarm with either key fob, cannot open door with valet key. Got into passenger side door put key in ignition and car won't start and I cannot fully close door cause window won't retract inside the door jam. When this happened two weeks ago i had car towed to local mechanic and he said battery was fine and he just had to reset the brain in the computer. Car has worked fine for a couple of weeks and then alarm goes off again middle of the night. Alarm is no longer going off but is temporarily in a no start mode. Can i just replace battery without having to reset the computer or are am I locked out of getting this started again with just a battery change. Any suggestions short of towing back to my dealer on what I can try to get this working? Thanks in advance.