Latitude D810 - mute button problems

My mute button has developed a mind of it's own. It will randomly mute & 'unmute' itself in rapid succession and this will continue until I repeatedly press increase or decrease volume. Needless to say, very annoying. I have checked and rechecked all drivers that I know of and they are fine. I cannot find anything relevant on the web at all concerning this problem. During my search, however, I did discover that by holding the mute button down and then powering up will cause an automatic pre-boot assessment. This probably explains why my system automatically began booting with this assessment test at random which is irritating. The only reason I can come up with is the button is physically defective. Does anyone know how I can disable the mute button altogether? I am using XP Pro with sp3. Any help will be appreciated. Thanks in advance.