To check for file alteration/corruption of program files, a “checksum” of a file is a much better detector than merely looking for a change in the file size. This is because it is possible for viruses to hide in empty spaces in a file or make internal functional changes without changing the file size (for example, by overwriting part of the file). Unfortunately, with the new breed of “stealth” viruses, even making a checksum of the file is not a guarantee that changes to the file will be detected.

Before concluding this chapter, we once again remind you to treat these symptoms as only a guide and not draw hasty conclusions from a matching symptom. It could very well be that the virus you are so sure has infected your computer, is actually a hardware or software related problem. So, don't jump the gun and howl “VIRUS”!