You don’t have to sysprep Windows 7. You can’t, in fact.
We don’t have a volume license for Windows 7; just a bunch of boxes that have individual licenses from Dell. We don’t use Ghost or Ghostwalker so SID-wise, I had to sysprep. Then change the license number after the unit is deployed. I did sysprep 7 and have deployed over 500 units so far.
http://theitbros.com/sysprep-a-windows-7-machine-%e2%80%93-start-to-finish