The quick solution is to run the secret: "net stop beep". That stops the secret beep service from running.
But a more permanent fix is to go into the Device Manager (Control Panel -> System, Hardware tab, then the Device Manager button). In the View menu, pick "Show hidden devices". Now down in the treeview, under Non-Plug and Play Drivers, there is a "Beep" driver. You can change the properties on it to disable it and stop/disable the service. Ignore the prompts to reboot now.
That's it. No more beeps. Well, unless you run VMWare. See my other post about that.
Find out what's on Clearance at ThinkGeek!