Restore disk

I bought an Asus CM 6850 used, on E-Bay, It came with no restore disk. I went on the Asus home page and they more or
less said give us your Registration number an $59.00 and if we feel you are entitled to the disk we will send it
to you or will refund your money, they refunded the money and no disk.Have no reason why, its all legit.
Is there a way to make my own. I know I did it in the past with other computers. HP and Acer but never with Asus.
T.I.A.