probably found fur porn on your hard drive and thought it was a virus

Well... FA did had those adult theme ads a while ago.. but sure it didn't came from that. o: maybe downloaded a flash thing or somethin from Fa and somethin attached itself to that download?

I haven't downloaded anything off of FA for months

webpages "download" things all the time needed for anything that automatically has to load up on any page you go to.

do you have adblock blocking all the newly added ads? if so it can't be that then.

it could also be a lack of proper security on FA. anything that has access to posting "whatever they want" as part of their ads (without first being properly checked by the admin team, since all they do is make sure the "source" of the ads is "trustworthy" and not actually know what all the ads are) could TECHNICALLY get past security faults in FA's programming (and there are many. a hacker friend of mine found several exploits in FA's code that still exists to this day, and sent an anonymous note to dragoneer about it, which never got a reply...) and force popups etc to load automatically when you load any page on FA. Again adblock should prevent that, but I can't be certain if some things aren't being blocked if they're not visibly there to confirm they're being blocked.

what kind of anti-virus do you use? and it is likely that its checking FA and finding an issue with its certificate.

Windows Defender

In which case I am not suptised. Get a few updates done and see from there

It's up-to-date

alright. as far as I am aware, FA doesn't have and viruses that could be leaked through. but there is a secondary kind of virus that makes it look like your hard drive is being scanned or look like an anti-virus software. the biggest downfalls to those is that they scan the computer way too fast.