Extreemly discreet for a virus
Some people have programs they put on your pc just for laughs, i remember one that removed all my icons on even days (the 9th had icons, the 10th no icons, the 11th icons etc...)
I also got one that moved my mouse away from the start button, each time you went close, your mouse *magicly* moved back.
Anti-Virus programs normally find them, even if they are not *nasty*
Next time, check what name your AV program gives it, and google the name