Your system may have a virus if you notice odd or unreliable behavior with your computer. This doesn't "prove" that your computer is infected, but it does indicate something that needs attention and perhaps intervention. Often the search for an infection leads to finding corrupted or damaged files or applications, or even incipient system problems, which have nothing to do with a virus infection. You know your computer better than anyone else, and if it begins to operate in a way that seems "wrong" to you, you should start investigating.