Malicious computer software, like viruses, are used by hackers for a multitude of reasons. Some just want to have fun, others are searching for personal data to use or to sell, still others are trying to make financial gains. Whatever the reason, a virus on your computer means trouble.

So, you might be wondering how to know if your computer has a virus? Here are ten possible signs that may demand your attention:

1. Running slowly

Viruses are actively doing something in the background on your computer. Whether that means sending messages or running ads or running a browser, there is always something going on. This added activity can slow down your computer a lot.

A slow computer is not a sure sign of a virus, but it is definitely a possibility. If your computers are managed by an IT service provider, they can inspect the systems more thoroughly and offer a detailed diagnosis.

2. Random shutdowns

If your computer continuously shuts down or freezes randomly, this is another indicator of a virus. Computers will often crash as a result of a corrupted file. The computer, upon trying to use the corrupted file, will attempt to fix the issue itself, but won’t be able to. This is what causes the force restart or shutdown without warning.

3. Popups

Popups are fake ads that try to convince you to click them. When clicked, these ads can make your computer vulnerable to other viruses. Sometimes, the ads will also encourage you to fill in your personal information. You can usually sense that the popup ads are fake, and thus likely to be a virus, because they are often too-good-to-be-true offers. If you start getting these types of popups, you’ve most likely got a virus.

4. Contacts receive phishing messages from you

One of the things a virus can do is to send out messages from the emails and social media accounts of its victims. This is to try to gain access to other computers by posing as a friend or a contact. The messages usually are very short and include a link. They may say something like, “I can’t believe they got this video of you!” and it will look like a YouTube link is attached. However, if that link is clicked, that contact will be vulnerable to a virus as well.

5. Loud/active hard drive

Since viruses are often working in the background, they can put a lot of stress on a computer’s hard drive. This is an especially noticeable sign of a virus when the computer is not being used, or when no programs are being run.

6. Missing or extra files

If your files start pulling a disappearing act, either by disappearing completely off of the computer or appearing in strange places, this is a sign of a virus. The good news is there is a chance you may be able to recover the files that have been deleted by the virus, but make sure to deal with the issue as soon as you notice something is amiss.

While the virus may delete files, it may also add files to your computer. These can be any unwanted files, and often include copies of the virus itself. If random files begin to appear on your computer, the most important thing to do is to not open these files, and to take the proper steps to get rid of the virus as soon as possible.

7. Strange activity with the browser

If your browser homepage suddenly changes without you having done anything, there are new browser ad-ons, or the browser is generally running very slowly, these too are good indicators that there is a virus on your computer.

8. You lose control

If the computer starts opening and closing programs on its own, or doing other things without user input, then there is likely a virus on the computer. The computer may also become unresponsive to action. For example, if you click or scroll and nothing happens. On the other end, it may click or scroll at will. If your computer seems like it has a mind of its own, it’s definitely time to check for a virus.

9. Error messages

Even if you are well-protected, some viruses are designed to specifically thwart your efforts. As such, if your firewalls or antivirus programs start acting up – namely, not opening correctly, or not installing correctly, you may have a virus.

Your computer might actually be able to tell you something is wrong. If you start getting popup error messages, try searching them to see what they are for. You might actually be able to diagnose the virus in this way.

10. Nothing is happening

Unfortunately, you may also end up with a virus that sneakily does things without you noticing. It is a good idea to scan for viruses every so often to ensure that your private information remains private. Protect yourself well with antivirus software and firewalls, and be diligent about the links you click and where you are putting your personal information.