Hard drive Firmware Password

i have a imac 21.5" and i have a firmware password on it and dont remember setting one its disabled all the commands when you turn the imac on i cant reset it from a disc without this password i cant find what model it is or what its running...just wondering if i just got a new hard drive (internal) would this fix the problem and i can just boot it up from a osx disc and if so how do i know what kind of hard drive ill need to get or is there other ways to fix this..thanks guys