Okay, I ran into this issue and googling it did not solve my issue. So I hope this helps someone else out.
We had a ups zp 450 printer and new windows 8.1 pc.
First tried the drivers from ups at http://www.ups.com/content/us/en/resources/ship/create/labels/technical.html
I ran into the issue where it would not install the drivers and the error was driver was not digitally signed. Phoned ups support and they said it was a Microsoft issue and directed me to a youtube video here titled 3rd party inf not digitally signed:
http://www.youtube.com/watch?v=dlYGfJM2BJ0
While I did not perform the task I was already familiar with temporarily disabling the digital certificate checks.
I did not believe the ups support that this was the only fix. So I went to the manufacturer website here:
https://ups.zebra.com/UPSZebraWeb/support/product-detail-ZP450.html
You have to create an account but eventually you can download the drivers from them.
I ran their install and it worked like a charm.
Hope this helps anyone having the same issue.
Regards,
Arnel