I occasionally use a VM for experimentation, knowing that I can safely trash the OS without affecting my day to day Windows installation. My PC isn't really up to running the VM though.
WUBI solves the problem partially as it allows me to boot into an Ubuntu installation that is stored on a filesystem entirely within an NTFS file. It runs natively and is completely distinct from my primary Windows installation but didn't require me to partition my drive.
Is it possible to dual boot any other operating systems in this fashion? Ideally I'd like to run a second instance of XP from a file within my main XP installation "WUBI style", instead of inside a VM.