Hi. First off, thanks in advance for any help you can provide.

Every time I start up my computer, I get the following message:

"Application failed to initialize: 0x800106ba. A problem caused Windows Defender Service to stop. To start the service, restart your computer or search Help and Support on how to start a service manually."

Then my internet connection is knocked out, then I get the following error message:

"ZeroCfgSvc MFC Application has encountered a problem and needs to close. We are sorry for the inconvenience."

Again, this happens EVERY time I start my computer. It's happened ever since I started using Roxio, which I subsequently tried to uninstall. I don't know if it's adversely affecting anything, but it's a nuisance, slows down my computer, and worries me that eventually it could lead to something worse.

Like I said, I'll be very grateful if anyone can walk me through the steps to remedy this.

This looks like apotential fix for you ,but you need to check the particulars of your PC with the one here.
See Recommendation :
Roxio support

I had the same exact problem same message numbers everything so i had to uninstall the old one and install a fresh copy that solved it for me hope it does the same for you.