Swapping OS on the fly

Only tool I know that does anything close to this is something called HyperOS but its not free, their website is http://www.hyperossystems.co.uk/ - it doesn't have them all running at the same time as that would be stupid waste of system resources - running one system is hard enough for your PC without three of them using the memory and processor power. I've never used Hyperos but i've heard of it.

Virtual Desktops is not a switch between operating systems, that has nothing to do with what you want. Virtual Desktop applications just hold windows and stuff in memory - its a far cry from an operating system.

You can run an operating system from inside another operating system through a virtual machine, which is basically the same thing as an emulator. If the virtual architecture is the same as the host architecture (for example, running x86 on x86) there are optimizations that allow the virtual machine to run about as fast as the real thing. There's only one operating system that's really running natively, but you can still dance around from OS to OS without having to restart.

I think Virtual Desktops are good to use but you should have a decent system to use virtual desktops because they take RAM and processing power but it has features that an original OS couldn't like taking sticky notes and minimizing a window to the system tray and customize a lot of other stuff.

The best virtual desktop in my opinion is probably Window Blinds. You could get it in http://www.stardock.com/

I haven't tried them out very extensively, but I'd try QEMU (free), as it reports close-to-native speed when used with the QEMU accelerator (also free to use). The price is right anyway.