You need one of the compatible rental xFi gateway devices: see https://www.xfinity.com/support/articles/getting-started-with-xfinity-xfi-advanced-security.
Although they are portraying xFi Advanced Security as a complete replacement for the Norton software they dropped (see https://www.xfinity.com/support/articles/norton-security-online-sunset), that's a gross exaggeration.
For example, Advanced Security cannot detect threats that arrive in encrypted ("https") data streams. Only software running on the customer's computer or mobile device where the stream is decrypted can do that. xFi Advanced Security is more like supplemental protection to AV software, not a replacement.
Users should install an antivirus program or app to take the place of the ones Comcast no longer provides.
If your desktop has Windows 10, it has Microsoft Defender Antivirus built into it.
I totally believe in users having to protect their systems the best they can.
Somethings that need to be taken into consideration is what their budget will allow, what type and amount of protection they need (based on their usage of the internet) and also their systems - users need something that plays nicely with their systems (does not slow down or limit performance).
There are other considerations, so I encourage all users to do their research and read as much info as they can on the pros and cons of their choices for protection prior to selecting what is best overall for their use.
Just as a starting point since Microsoft Defender Antivirus was mentioned previously for WIN 10, I thought the following article woulod be a good starting point for users' research as it mentions quite a few different options and pros and cons of each.
More info here:
It appears to me that Safety Detective profits off the sale of Norton and other antivirus programs. They can't profit off of Windows Security with Microsoft Defender Antivirus because it's included in Windows 10. They can also force their tests to create a desired outcome that's in their favor.
I've been using the protection offered by Microsoft since 2007, and none of my computers have been compromised.