Since a virus is technically an unwelcome software program that is installed on your computer, the way it affects your computer will vary depending upon the virus program as well as your operating system. In fact, if the virus is very well programmed, you may not notice any signs of infection at all. Furthermore, some of the signs that are commonly associated with a virus infection may actually be caused by something else. Still, some of the signs that are most commonly associated with a virus include: Your computer starts running more slowly than usual
Your computer stops responding to you or starts "locking up"
You start receiving unusual error messages
In short, your computer simply stops acting normal.