Originally Posted by Chris Down
Go to Start, then type 'regedit' into run, or into the search form in Vista. Then go to HKEY_CURRENT_USER\Control Panel\Sound by clicking on the plus buttons next to the folders to enter that folder. In that folder should be a key called 'Beep'. Double click it, and change the value to be equal to no. Now, when you reboot your PC, all the beeps should be gone.
It's easier (and safer for new computer users) to go to the Control Panel / Sounds and Audio Device / Sounds / Sound Scheme / No Sounds.
Saves the new user the trouble of asking how to fix the registry on the forum...