For a quick solution, install a remote tool like Teamviewer onto her office PC, so that she can still work from home. For server access and if you want tight security, it's better to install a hardware firewall with VPN feature. In this way, you can control who can gain remote access back to your office.

There are differences in installing a free anti-virus vs paid ones.

First, free anti-virus is standalone. You won't know if your colleague PC is infected with a virus. Very often, a free anti-virus bundle with other third-party bloated software or adverts during the installation. It installs non-related software and tends to slow down your PC performance.

Why? The developers that create this free software needs some form of revenue to support their project. Also, they’re not obliged to provide the latest virus protection and immediate support when your PC is infected. If budget is the main concern, Windows built-in standalone defender works okay though.

The main reason why business still buys paid anti-virus is for the support and latest protection. Most importantly, you get to see an overall view of all PC health in a single dashboard.

It's a great decision to backup your office PCs. Unlike a NAS or server, your PC has probably only one hard disk. That means if it crashes, your data is lost too. Though business PC offers three years onsite warranty, it does not cover data recovery if your hard disk fails.

Firstly, it's recommended to use a centralised NAS storage to store every PC data, over a portable hard disk attached to every PC. It's overall cheaper and easier for any engineer to monitor if a backup fails.

Now, PC backup works best if all your PCs are linked to your office network mostly via wired, and not wireless. Why? Because PC backup will transfer the data across your office network and uses high network bandwidth. This network consumption results in slower internal network speed, which in turns affects a slower Internet and share application and files access.

No matter how good backup software is, it consumes PC resources when the backup kicks in. Hence, you'll experience PC lagging. It's important to plan the backup strategy at staggered timing during off-peak hours like lunch hours. Also, a good backup software should give you the flexibility to backup specific files at scheduled timing, and provides notification if the backup fails. I do not see any need to backup the PC operating system as you can recover them from the manufacturer recovery image.