You can find your authentication code by

If you can boot the system
Control panel --> system Icon --> ProductID

If the system cannot be booted but you have not deleted the files, you may
try booting the bootdisk.

At the A: prompt, type:  C:\Windows\Command\Find /N "ProductID"
C:\Windows\System.dat   (Note:  ProductID must be typed in upper and
lower case letters as shown and there is a space after:  "Find"
after:  "/N"    and after:  "ProductID"