Well, maybe you don't.
Many people just think that they have a virus, because something is wrong.
These are not sure signs of infection. The first step to take to check for virus infection, is to use a reliable virus checker. However, no matter how much they tell you how good they are, I recommend using at least two of them. After you have installed them, run them to see if they find a virus on your computer. If there is only one infection, it may be a false alarm.