If your computer starts acting visibly funny (as apposed to funny in the sense of weird processes or network activity), you can almost entirely rule out a virus, those like to stay hidden.
That's an interesting observation actually, because it didn't use to be the case. It used to be the case that most viruses would intentionally make a nuisance of themselves or display messages every now and again (e.g. you might get a message on boot every 10 times or so - too often and they would get eradicated before they spread).
Viruses that are intended to stay hidden and undiscovered is a new thing, relatively speaking.
Viruses that are intended to stay hidden and undiscovered is a new thing, relatively speaking.