Instead of dual boot, have you looked into virtualization? The user can continue to use Vista as the main OS, but instead of going through the trouble of rebooting to use another OS for one program he can just open a virtual machine.
VMware has a free client and is easy to use. You can get pre-built virtual machines
here. (Even though it's "linux box" you can install any OS.)
Virtual Box is another good free alternative and doesn't require the use of a pre-built virtual machine.