I once had some package that you could run to instantly replace a running Windows instance with a running Linux instance. I'm not talking about virtualization or coLinux. I'm talking about the moral equivalent of hot-swapping out the Windows kernel and replacing it with a Linux kernel. It may have only worked on Win9x for all I can remember. But I haven't been able to think of the name or find it since I happened upon it many years ago.
I think it's GRUB4DOS using Win9x's option to start DOS applications in compatibility mode