Computers are complex enough machines that they don't always do precisely what we expect. Sometimes a sudden and unexpected behaviour is just a fluke; at other times, it's an outward and visible sign of an inward and terrible malware infestation. If you notice any of these security warning signs, your system may well be compromised.
1. Popup ads
Look out for popup ads appearing even when no browser is open. While not as common as they used to be, adware programs bombard their victims with advertisements. Sometimes they're ads for legitimate products, but they can contain links to malicious websites – sites that will attempt to drop more malware on your PC.
2. Browser navigation gets redirected
Not every site redirect is malicious, but if you find that trying to reach Google takes you to an unfamiliar search site, you've almost certainly got a problem. Sometimes the redirection is more subtle. For example, a banking Trojan might divert your browser to a fraudulent site that looks just like your bank's real site. In that case your only clue is the unfamiliar URL in the browser’s address bar.
3. Fake security program warnings
Look out for scary warnings suddenly appearing from security programs you don’t ever remember installing. Creating and distributing fake antivirus programs is a lucrative business. The perpetrators use drive-by downloads or other sneaky techniques to get the fake antivirus onto your system, then display scary warnings about made-up threats. Naturally you have to register a payment before the fraudulent tool will "fix" the problem. And of course scanning for malware with the fake AV is super-fast, since it's not actually doing anything.
4. Posts you didn't write appear on social media sites
Malware focused on Facebook and other social media sites propagates by generating fake posts. Typically these posts include an inflammatory statement of some kind to try and pique curiosity and get folks to click on it, such as "OMG were you really that drunk? Look at this picture!" Anyone who falls for the fake and clicks the link will become the next victim, and so the infection spreads.
5. A program holds your PC for ransom
Some malware programs literally hold your PC or data for ransom. Overt ransomware, as it’s known, may encrypt all your pictures and documents and demand that you pay to get them back. Other forms try to obscure what they're doing. For example, they may display a warning supposedly from the government stating that your computer was used to send spam and demanding that you pay a fine before you're allowed to use it again. Of course, even if you do pay, you may not get your system back.
6. Suddenly you can't use common system tools
A smart user, suspecting the presence of malware, might launch Task Manager to investigate, or check settings using Registry Editor. If you suddenly find that trying to use these or other system tools triggers a message saying your Administrator has disabled them, it may well be an attempt at self-defence by malware on your system.
7. Everything seems perfectly normal
That's right. Some types of malware do their best to hide all activity, leaving no visible traces. Even when you don't notice anything unusual, it's possible that a bot on your system may be quietly awaiting instructions from its command and control system, or a Remote Access Trojan may be harvesting your personal information.
If you think that malware has taken up residence on your PC, install a powerful antivirus utility or security suite immediately (BitDefender is one example, as the suite scored a full five stars in our review). Already got one? Then apparently the malware got past its protection. Make sure your antivirus is fully up to date, and run a full scan.
Also, get a second opinion from a free clean-up-only antivirus like Malwarebytes Anti-Malware or Comodo Cleaning Essentials. You definitely want to get that nasty, malicious program out of your system as soon as possible, before it invites "friends" to make your security problem even worse.