Hi there, Laserfish.

Thanks for letting me know that you've tried to reinstall QuickBooks Pro 2017. 

You can try to clean install your QuickBooks Desktop. It'll removes the recent operating system as well as the user files during the installation. It works better to fix company files issues: You check check out the clean install QuickBooks Desktop article to get more details with this.

You can try logging in to your system as Admin user.  You can also check out the internet connection setting on your machine. Then, you'll have to open Internet Explorer (IE). Once internet is connected, try to download the update. Here's the article that you can check out for further details: Configure  Internet Explorer to work with QuickBooks Payments and related applications.

You may also wan to check for additional solution that you can use: Configure firewall security settings for Desktop.

Let me know how the solutions work on your end. I'll be right here.