Mute switch.
Or, more permanent:
Start, Control Panel, Sounds and Audio Devices,
In the box look for Windows then Start Windows... it has a tiny speaker icon next to it. Highlight Start Windows, then on the pull-down menu below it, select "None."
Or, turn off all Windows sounds in the Sound Scheme pull-down menu.
Click Apply, and you're done.
If you want, you can substitute any audio clip that you have saved on your computer for any of the normal Windows sounds. I once had some clips from Monty Python that played at appropriate times. I also used Homer Simpson's "Doh!" for the critical fault sound.