Strictly speaking the O/S should be removed from the PC before the sale unless the disk are also supplied.
Many people though including small retail builders will install an O/S so that they can test the system themselves and leave it on for you to see all is working correct when you first receive the machine.
You will need the disk for some installations of peripheral hardware and to re-install corrupt drivers/files unless the i386 folder has been copied to the drive so although left on the machine it will not be of much benefit.
So I suppose the answer is legally no but I don't think Microsoft are going to jump on anyone doing this for the sole purpose mentioned above. If the disk has been imaged to a partition of the drive then this will be a different matter altogether.