Sounds like he's inquiring about Parallels or Fusion and wants to go from one to the other without rebooting.
SaltDoc - The latter is true...
You can install windows on the bootcamp partition, then install fusion of parallels under OS X. Both products can open the bootcamp windows install as a 'vm' giving you the exact option you describe above.
..... it seems you have to "activate" it again when you install the parallels/fusion. is this possible to do with an oem version of the OS?
You won't get the same level of performance running in a VM and the full capabilities of the graphics card won't be available. Not sure there's much else significantalso, i have seen it said that some features of parallels or fusion may not be able to work when you install the os through bootcamp.... anything important missing?