False positive caused by hoax programs
If it happens to you, that your computer starts to behave unexpectedly, even suspiciously, it need not be any virus, which is behind it. It may be a prank program, installed in your computer by your colleague, or which was presented to you with false or misleading information on its purpose. An example may be the installation of the presentation of "blue" pictures at the computer start-up. People of weaker characters or less experienced users may have problems with placing into original status, and they may consider a similar "prank" to be caused by the effects of an especially treacherous virus.
Nevertheless, it may not be true. And how to recognize a "hoax" from a real virus? It may be difficult as it is not possible to specify the exact borders of these two groups. It is necessary to base your analysis on the particular situation. For example, viruses cannot afford presentations of some graphical images (especially in colors) because such pictures are too large. The most important difference, which, however, is not easy to recognize, is that unlike "jokes", viruses are reproducing.