Virus, spyware and other types of malware are here to stay. Our main problem is becoming a little paranoid about the subject. One of the reasons that drives to this situation is that many people–even skilled ones–are not aware of the symptoms of having a malware infection.
This post is just a basic list, I am preparing a more advanced article about the same topic so take this one as an introduction.
The best way to know if your computer is infected by some sort of malware is by using an antivirus to detect infections. So, what is the point in this list? Sometimes, the security utilities have problems detecting some sort of virus, as happens with the more sophisticated viruses out there.
Slow performance could be seen a sign of infection, but as there are so many reasons that can drive to this behaviour is not included in the list.