Thanks for the replies. I strongly suspect it was a false positive. Defender and a few others flagged it, but the major antivirus programs did not. The alerts are triggered by heuristic analysis of the 3.0.0 standalone and 3.0.2 portable, but not the other versions I tested. I also did my homework and went through the steps to verify the signatures - an essential process, I agree, but it really needs a simple, intuitive console for the uninitiated.

Two weeks ago I downloaded Electrum 3.0.0 Standalone from Electrum.org.
Worked fine until now, but today when I tried to copy the file from my laptop to a USB drive Windows Defender identified it as containing Trojan:Win32/Bitrep.A virus. It then also quarantined the file on my laptop as well. Apparently other users are reporting the same issue.

Three immediate possibilities come to mind, 1) the source file was infected, 2) my computer became infected after downloading it, 3) it's a false positive. While it's probably #2 or #3, have there been known cases of malicious code appearing in .exe or .asc files on the electrum.org site?

(edit: I tried downloading the Electrum 3.0.0.exe again from the site and got the same virus alert. However, Electrum 3.0.2.exe scanned clean)

Is there a step by step guide for dummies on how to verify the signature on the .exe file, or a simpler way? The instructions I've found so far are partial or daunting and incomprehensible.

And is there a safe way to transfer my bitcoins in the infected wallet to a new wallet?