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[ The PC Guide | System Care Guide | Data Loss and Virus Prevention | Virus Detection and Protection | Virus Scanning and Antivirus Software ]

Symptoms of Virus Infection

There is usually no simple way to know when you have a virus. I should rephrase this: some viruses exhibit behavior that tell you immediately that they are on your system, but so many viruses mimic other system problems that it is most accurate to say "there is usually no way to rule out a virus as a possible cause of strange software or system behavior on your PC".

Virus writers are usually pranksters whose software is normally designed to avoid detection, and to cause mischief. Both of these mean that viruses are designed to mimic natural processes in your PC so that you don't think you have an infection. They often make your system behave in strange ways, but in strange ways that could also be the result of an application bug, driver glitch, or even a hardware problem. For example, a virus can make strange things print on the screen--but so can a driver problem, or a bad BIOS setting. A virus can make the system spontaneously reboot--but so can a bad power supply, or an operating system problem, or an overheating processor.

Since viruses are so common, and they can exhibit so many strange types of behavior, and so many of these behaviors are similar to other, real hardware and software problems, I always recommend a virus scan as the first step in troubleshooting a hardware or software problem. Until virus infection has been cleared as a potential source of problems, it doesn't make much sense to look for a real hardware or software cause. For example, many symptoms of resource conflicts are similar to the symptoms of virus infection. The following are the sorts of problems that are usually real system or component problems, but can also be a result of virus trickery (this list is not exhaustive; virus writers are very creative):

  • Spontaneous system reboots.
  • System crashes/hangups.
  • Application crashes.
  • Sound problems with the speaker or sound card.
  • Seemingly random glitches on the screen.
  • Corrupted hard disk data.
  • Partitions that seem to "disappear".
  • System slowdowns.
  • Hard disks that won't boot.

There are some types of strange system behavior that hint much more strongly that a virus is around and responsible for the problem. Seeing any of the following types of behavior on your system should send you scrambling for your clean antivirus boot disks, as they are not normally caused by legitimate hardware or software problems:

  • Strange Messages: If you boot your PC some day and are greeted with the message "Your PC is now Stoned!", then you can bet your booties that you have a virus. Oddball messages on the screen usually are viruses (although some real system messages are kind of strange too, such as "No ROM BASIC - System Halted").
  • Odd Text Games: If you type at the command prompt in DOS and the letters start moving around on the screen in strange patterns, or each letter you type is changing to a random color, or ASCII graphics move around the screen "eating" other characters, or anything strange like that, chances are high that you have a virus.
  • Music and Strange Sounds: Viruses have been known to generate music or odd tones on the system speaker.
  • Changing File Sizes or Time/Date Stamps: File-infector viruses commonly increase the size of files that they infect, and some can affect date/time stamps as well. There is normally no ordinary reason for an executable file to increase in size (unless you apply a patch to the program or something similar.) Time/date stamps are more easily changed, but still normally remain static over the life of a program.
  • Disappearing Files: If you used a program yesterday and now it isn't there any more, and you're sure that you didn't delete it, a virus may have deleted it for you.

Next: Viruses in Memory, and the Importance of the Clean Boot Floppy


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