Fusion, as Parallels, does not required to reboot the computer; it is based on a virtual machine:
Love your Mac even more with the ability to simultaneously run any PC OS—Windows, Linux, NetWare and others—on Mac OS X. Switch between operating systems by easily tabbing between applications and share data between the two operating systems by dragging and dropping files on the fly—all without needing to reboot. What's more, you can create virtual machines and run them on other VMware products or run any VMware virtual machine on your Mac.
To register and get additional info: VMWare Mac.
