I am aware of this question: Help! My home PC has been infected by a virus! What do I do now? and would adhere to its answer if I had, as this question states, "determined beyond doubt that my home PC is infected by a virus".
However, I haven't determined this. Since I believe this can easily be a generic question, let me say what happened:
- I followed an outlink from Wikipedia to a legitimate looking site
- The legitimate site loaded however, after a few seconds, it changed to a clearly fake warning that my Firefox was out of date, prompting me to download & install a new version (which, in reality, was a suspicious file from Dropbox);
- I left the site without installing the likely malware.
However, now, there are basically two possibilities; either the website got hacked or I have a browser redirect malware installed on my computer. I would assume the former, since I've already have seen legitmimate websites showing a piece of scareware, and this even once happened when I had followed a Wikipedia outlink, BUT... After seeing this fake Firefox download today I tried browsing that suspicious page, reasoning that nothing worse that has already happened may further happen... AND I never saw this scam again. This perplexes me, since all other legitimate-looking websites hacked to show scams were showing them consistently. On each visit. So either this particular website I visited today dealt with the problem minutes after I left it, or the scam is designed to never show itself to one person more than once, or... I have a browser redirect malware installed.
Maybe I shouldn't, but... Since I was unable to find this scam on this website again, I was browsing and working normally for a few hours hoping to spot any irregularities. I didn't spot any.
Since I believe it would be an exaggeration to nuke an entire system from orbit just because a website shows me a scam, how should I make sure that such extreme measures are or are not necessary? I've run a full scan with Windows Defender and no threats were found.