How best to clone

Hi – I am the sole proprietor of a small PC Builder and Reseller business in North Western Ontario.
When I build 10 identical machines I am forced to install the same software on each as the product codes must be unique. Otherwise we get in trouble from software companies.
This is very time consuming.
How do I change the product key of various sofware so I can Clone the “perfect hard drive” and lateer input the codes off the actual disks that the end-user will have?