How do I remove my HDD password?

I have an old Acer Travelmate 2440 laptop. About 6 years ago when I got it I put a "primary master HDD password" on it. when it started to get old i set it aside. Well, now my son wants a laptop and I wanted to let him use it as a trial before I get him a real laptop. Only thing is, I forgot the password. I took it apart, removed the CMOS battery and let it set overnight. next day, I put it back together and it still wanted the password. How can I remedy this; if its even possible? I'm mechanically savvy so i'm good at taking things apart and putting them back together, I'm just not that adept at computer functions.