Online security firm AVG has released a list of the top 10 signs that could mean your computer is infected with viruses, malware or Trojans.
The firm warns that many of today’s threats are designed specifically to go undetected.
But there are some obvious signs to look out for when a system has been infected.
"Even if you don’t know about security issues, there are things to watch for,” AVG says.
Here are the most common symptoms of infection:
My computer is running unusually slowly
If a PC has been infected by a virus, worm, Trojan or some other form of malware, it could be running tasks that consume a lot of resources, making the system run more slowly than usual.
My computer keeps popping up messages
If pop-ups and messages appear on the desktop, either advertising things or saying that the PC is infected, the computer probably has a malware infection. Spyware or adware is also commonly used as well as fake anti-virus programs (also called "rogueware”) to deceive the user into parting with financial details.
When online, my PC displays pages I‘ve not requested
Many threats are designed to redirect traffic to certain web sites against the user’s will, and can even cause browsers to display spoof web pages. Normally this happens when looking for something in a search engine or when typing a URL into an address bar. The user presumes they have opened a legitimate website when really they have been taken to a malicious imitation. The free AVG LinkScanner on Windows or Mac computers will help protect from these web pages.
I cannot connect to the Internet or it runs very slowly
Loss of internet communication is another common symptom of malware infection. This problem could also be due to the Internet service provider (ISP) or modem router. If a computer has been infected, the malware could be connecting to a URL or opening separate connection sessions, thereby reducing available bandwidth.
My anti-virus has disappeared, firewall disabled
Another typical characteristic of malware threats is that they disable the security software already installed on computers. If one security protection layer shuts down it might be a specific software failure. However, if all security components are disabled, the computer is infected.
Applications won't start, PC keeps crashing
This is when infection causes applications from the start menu or desktop to stop working and other programs to run. This could either be caused by an update or poor installation process or if the computer unexpectedly or erratically crashes persistently then it may be due to malware. This especially happens when trying to open up a security program, or trying to fix malware problems.
My computer has new programs
If there are new programs on the computer’s desktop that have been downloaded and installed without consent or knowledge, it's likely the computer is infected. Usually these programs have explicit or illegal content and it's near impossible to delete them like a normal program.
Where have my files gone?
There are malware threats designed to delete or encrypt information. Others move documents from one place to another. Some remove the library files for running games, security software and other programs. If a computer is in this situation, it’s highly likely to be infected.
My computer is speaking a strange language
Certain infections cause the language of certain applications to change or the screen to appear back-to-front.
My computer has a mind of its own
If the computer starts acting on its own or sends emails without your knowledge, it is infected. Other signs include internet sessions or applications opening sporadically.
If you notice any of the above on your machine, take a look at AVG's free malware removal tools.