The only way to get Windows 7 would be to purchase it and then install on your machine. You might run in to all sorts of Driver problems too.
What money back policy does the Retail Outlet have? Perhaps there is a 30 day money back type of refund if you are not happy with the purchase?
Is purchasing a Scanner compatible with Windows 8 an option for you?
See Control Panel\All Control Panel Items\Troubleshooting\Programs\ Program Compatibility Troubleshooter
Windows 8 Compatibility Center
What do you mean by "All Documents/Internet Options"?
Have you tried right-clicking on the shortcut to the program, or the .exe, and selecting the "Run As Administrator" option? Do so even if your user is an Admin.
Try right-clicking on the actual Setup.exe and selecting the "Run As Administrator" option? Do so even if your user is an Admin.
This EXAMPLE (ONLY) might or might not work for you with your program it uses the built-in Administrator account.
"It seems that the "Run as administrator" command only gives you some administrative privileges, and they're not enough to install Java (and OpenOffice, by association). In order to get around this, you need to actually log into the built-in administrator. Open a Command Prompt (Admin) with admin rights by using the Windows Key + X and selecting that option. At the prompt, type:
net user administrator /active:yes
Then log out of your regular account and back in as the built-in administrator, where you might be able to install the update. When you're done, log back into your regular account and, for security purposes, disable the built-in administrator account using:
net user administrator /active:no "
Built-in Administrator Account - Enable or Disable in Windows 8
How to open an Elevated Command Prompt in Windows 8
