What's the context for doing this? If you're trying to control internet access on a per-user basis, you can write to HKCU during the login process as a part of your login script (but there are extra hoops you'll have to jump through if your clients are running Win9x). If you want to be able to remotely turn off access while users are already logged in, you may be disappointed, as I believe once a browser is launched, the proxy settings are read once and not re-checked for the duration of the browser session. And, of course, if you want to be able to do it while nobody is logged in, there isn't an HKCU hive anyway!

You can also control proxy settings through Group Policies, but again, don't expect a change to affect users who are already browsing at the time the change is made. Les is correct that you can remotely write the HKLM proxy settings if you set proxy per-machine instead of per-user, but it still won't affect users already browsing when you want to "throw the switch."

I don't know of a reliable way to do this for users who are logged in, so if it could work for you, doing it through a login script or GPO might be your best bet.

Joe