Any running app can put an icon in the system tray ("notification area" under XP) by calling ShellNotifyIcon().
Basically, you have to configure each app that puts an icon there not to put an icon there.
There are not necessarily services, though some of them are. Editing the registry isn't the way to go. The proper solution is to configure each app not to put an icon there.
For instance, the volume icon can be turned off in the Sounds and Multimedia control panel under W2K.