Cap maximum volume on Windows XP
Solution 1:
There is no way that I know of to limit the maximum volume on Windows.
I had the same issue at a library where I had to set a few systems up. In the end we put headphones on each machine.
Also, if your speakers actually have volume knobs on them, the user will simply be able to change this.
I would either use headphones or get some speakers, set them to what you want, and rip off the volume knob (but unless you put super glue on it or something, the user will be able to change it) - last being a extreme option!
Lastly, if these machines are "locked down", a option would be to set it to whatever you want, then add the volume control application to disallowed applications - but even through this method, scripts or other software that hooks directly into this function could overwrite the volume setting.