Updated September 9, 2020.
If you’re handling sensitive data, you need to keep your computer safe. Here are some inexpensive ways to do that.
First, enable full-disk encryption, which is built into Macs and Windows computers, using a reasonable password.
Second, visit Objective-See Software for some fine free preventive programs:
You can also set up a PiHole — a DNS-based ad filter which will prevent a lot of malware (and ads) from loading at all. This is usually done via hardware (a cheap efficient little computer somewhere on your network), but if you are really clever and only allow it to take requests from one IP address, you can also set it up on a cloud computer somewhere. The downside of the latter is (a) it's one more point of attack, and (b) when you leave your office, you have to switch to something else.
DetectX Swift is sold outright rather than by subscription, and a trial version available. It is a signature-based malware detector, but it also detects any changes to your system, making it easier to figure out when something funky has happened.
LaunchControl ($15 for personal license, trial available) is very good finding and removing startup software in places that are hard for novice users to find (not free.)
Malwarebytes can be installed without the monitor, for malware detection after the fact (free but they keep asking you to upgrade.)
Software which doesn't keep my computer, but does keep me sane: