What to do: Not to sound like a broken record, but typically random pop-ups are generated by one of the three previous malicious mechanisms noted above. You'll need to get rid of bogus toolbars and other programs if you even hope to get rid of the pop-ups.
Sure sign of system compromise No. 5: Your friends receive fake emails from your email account
This is the one scenario where you might be OK. It's fairly common for our email friends to receive malicious emails from us. A decade ago, when email attachment viruses were all the rage, it was very common for malware programs to survey your email address book and send malicious emails to everyone in it.
These days it's more common for malicious emails to be sent to some of your friends, but not everyone in your email address book. If it's just a few friends and not everyone in your email list, then more than likely your computer hasn't been compromised (at least with an email address-hunting malware program). These days malware programs and hackers often pull email addresses and contact lists from social media sites, but doing so means obtaining a very incomplete list of your contacts' email addresses. Although not always the case, the bogus emails they send to your friends often don't have your email address as the sender. It may have your name, but not your correct email address. If this is the case, then usually your computer is safe.
What to do: If one or more friends reports receiving bogus emails claiming to be from you, do your due diligence and run a complete antivirus scan on your computer, followed by looking for unwanted installed programs and toolbars. Often it's nothing to worry about, but it can't hurt to do a little health check when this happens.
Sure sign of system compromise No. 6: Your online passwords suddenly change
If one or more of your online passwords suddenly change, you've more than likely been hacked -- or at least that online service has been hacked. In this particular scenario, usually what has happened is that the victim responded to an authentic-looking phish email that purportedly claimed to be from the service that ends up with the changed password. The bad guy collects the logon information, logs on, changes the password (and other information to complicate recovery), and uses the service to steal money from the victim or the victim's acquaintances (while pretending to be the victim).
What to do: If the scam is widespread and many acquaintances you know are being reached out to, immediately notify all your contacts about your compromised account. Do this to minimize the damage being done to others by your mistake. Second, contact the online service to report the compromised account. Most online services are used to this sort of maliciousness and can quickly get the account back under your control with a new password in a few minutes. Some services even have the whole process automated. A few services even have a "My friend's been hacked!" button that lets your friends start the process. This is helpful, because your friends often know your account has been compromised before you do.
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