How to completely disable the Windows taskbar?

The only way to do this would be to completely and entirely stop using Windows Explorer, and use a custom shell to replace explorer completely. This means you would no longer have the usual start menu, explorer windows, etc, and would have to rely entirely on some other piece of software for browsing your file system. Some people do this, and there are replacements available, so if you really want to completely eliminate the windows taskbar, find and use one of these of your choice. There is already a SuperUser question discussing the various options.

In order to completely replace explorer, and have your replacement be the default shell which opens up when you log in, create a REG_SZ value called Shell in [HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Winlogon] using the registry editor. This will completely replace explorer for your account, making it unnecessary to use any taskbar hiding/killing techniques.

"Taskbar Magic" is the answer to ALL of your problems.

I had the exact opposite issue as you, my taskbar was gone (it's an embedded version for a hosted kiosk, but I was also using the machine for alternate reasons so I needed the taskbar back) and realized that Taskbar Magic is why I couldn't get my taskbar. Start menu still works if you hit the Windows key, and everything else still works fine, the taskbar is just perma-disabled.

I believe this is the main download site: http://rocketdock.com/addon/misc/3425

It also contains instructions on how to download and install it.

Try Taskbar Hider. You can put it in your startup folder and use a shortcut key to show/hide the taskbar. It works on Windows 7 64-bit.

You can still access the Start menu with the Win key, even when the taskbar is hidden.