Just to make sure: Photoshop 6 or Photoshop CS6?
Generally speaking, you'll want to reinstall the software from the original media.
it's Adobe Photoshopw CS6 Windows Full Edition-Electronic Download thats what it says on the invoice e-mail, does that help at all
With Ps CS6, first Deactive that on the old OS, before wiping the disc.
Then, run the downloaded installation file, after Extracting.
When installed, Activate with your S/N.
If you did not Register (separate from Activation), I recommend doing so now.
What Bill Hunt says, just remember you'll need to apply any applicable updates to Photoshop CS6 after installing it.