As you can read on the Official Parallels Virtualization Blog, with the release of Vista Microsoft has modified its EULA (End User License Agreement) to prohibit the use of Vista Home Basic and Home Premium editions with the products of virtualization like Parallels and VMware. Macworld USA has confirmed the information with a Microsoft representative.
This does not prevent Vista Home editions from being installed on Macs using Boot Camp, as Boot Camp is not a virtualization or emulation technology, but rather allows Windows to run natively on a Mac. For now However, Boot Camp remains in beta development phase, although it works officially only with Windows XP.
Meanwhile, the EULA included with Vista Enterprise and Ultimate editions allow such operating systems to be installed on top of virtual systems or emulated hardware.
As Rudolph writes on the blog, “what this ultimately means is that if you want to run Vista virtually, you must buy the more expensive versions of Vista, or you are violating Microsoft’s license to use agreement.”