This is one of the cases where using MSConfig is easier than managing the registry with a registry editor.
The rest of the instructions should still apply (namely stopping the providers), unless your are neither going to use Avast as a "second opinion" antivirus during the time of the test. Updates should be done manually. When you search for program updates and there is an update (even only a newer Virus definition files), the icon will be back (the providers will still be disabled), and you will have to use the MSconfig again. I believe that if the providers, and all real time avast processes are disabled, you may use another AV without conflicts, even if the Avast icon is in the system tray.
As for MSConfig, simply type it and press enter from the Start/Run box, and then after the MSConfig window is up, choose the "Startup" tab and unceck the "avast!" box.