I have a Mac running OS X 10.12.6 and was trying to help a friend with his Windows machine. He has a legitimate version of MS Office, but needed to reinstall it and couldn't find his product key, so I tried helping him get his installation activated. I ended up on
https://freemicrosofttoolkit.com/
and tried downloading from one of the mirror links. This was on my Mac using Chrome, but when the download finished, Chrome said it detected a malicious file in the download and removed it. Since the download was removed, I didn't unzip or execute anything that was downloaded, but since then, I've been noticing some strange behavior, with some programs freezing up, and most notably, I periodically see my entire screen flash briefly like a screenshot is being taken. If I'm plugged into an external monitor, both screens flash simultaneously. It doesn't seem to be on a set schedule (e.g. every 60 seconds), but it happens periodically. I downloaded Malwarebytes and ran a scan that came up clean, but I'm still suspicious that something is lurking in the background.
What other scans or steps should I take to look into this? Thanks in advance for any assistance.