Can I use OEM versions of Windows on my Mac?
Generally the answer is yes. Though if you purchased a computer with an OEM version of Windows then the license only covers that hardware and according to the terms of use can not be transferred to another computer.
These OEM versions will work with Boot Camp and/or Parallels or VMFusion. Though as OEM versions you won’t get any support from Microsoft, for that you will need to purchase the full retail versions.
Note that Windows Vista Home can not be used with Parallels or VMFusion due it licensing restrictions.