"will have to be installed within a pre-existing install of Windows"
Pertinently not, although in practice it will often be the case that
Windows XP will be the "normal operating system" installed..
Cloud (NOT "gOS cloud" as gOS and cloud are two separate things
altogether) acts as a "BIOS extension", and it doesn't ,matter what
else is installed on the computer, could be some form of Windows,
could be another full Linux distribution (even gOS).
The computer boots into Cloud, and Cloud can be used immediately, in
the background the computer (Cloud) invisibly continues to load the
"main" operating system, and once this operating system is running
Cloud presents a menu bar option to switch to it. All the while you
can use cloud to surf the web, and use web applications, and
applications built into cloud that run inside the browser.