Virus Scan.

One of the first things we do is kick off a Virus/Malware scan using

Malwarebytes Anti-Malware Free..
Run the Virus Scanner at least once a month or if the machine does anything strange.
Stick to the free version. It doesn't run in the background and interfere with performance or other programs. Click "I don't need real time protection" and "ok" when it asks.
Works on Windows and Mac computers as well as newer Phone and Tablet devices. Does not work on some older Macs, phones and tablets.
Deactivate Free Trial. Stay with the Free version. I do encourage purchasing a license too support the company, but don't install it. We have seen many machines slowed down by the paid version. Never Pay For Anything. Except us.😃

While it is running, we start a manual Tune-up.

In our experience, no one Anti-Virus catches everything. By running a seperate scanner, it forces your regular anti-virus to scan the files at the same time. Thereby catching a lot more viruses than an Anti-Virus program alone.