text: when you buy a pc pre-installed with xp are you paying for xp? it's just that you normally have to pay for a recovery disk to restore your computer.  when you buy a computer of the shelf you get xp with the machine pre-installed.  are you paying for that pre-installation when you buy the pc?  with an oem version of windows does the rule apply that you can only install that disk once on a machine or can you install that disk on many machines? okay, the difference here is oem  most oem installs of windows xp have a license on the side of the box.  microsoft knows the "cd" key number.  because most oem's are assembled with an un-keyed version of xp because the manufacture uses a motherboard that is "tattooed" has the "key" embedded in it.  let's say you want to put a new motherboard in the pc that is not the original to the pc.  and you desire to transfer win xp to this machine, first the restore disks will not work on a motherboard that is not 'tattooed', second (let's say you get a xp disk) you install xp and attempt to use your 'cd key' from the side of the box.  it will come up that you need to call microsoft.  and the short of it is, that most oem version's of xp can only be registered on the computer (motherboard) that it shipped with.  however, now if you bought a windows xp o/s and installed it on a computer, and now want to install it on a new computer, back up and copy the data you need to keep.  remove xp from the old computer and install it on the new one.