Symptoms and Effects 
It can be difficult to determine if your computer is infected with a virus, which is why it is necessary to have a reliable anti-virus software installed on your computer. There are however symptoms that you can look for which indicate the possible presence of a virus. A virus infection can cause the following symptoms, but keep in mind that these symptoms can also be the result of other problems on your computer.
- The computer shuts down and restarts for no apparent
reason.
- The computer takes a long time to start and runs very
slowly. This can be caused by having too many programs
open, problems with the network, but also by a virus infection.
- Sudden reduction in disk space or memory capacity. This
may be an indication of a virus infection as viruses can
sometimes consume all available free space.
- If the same file appears twice, one with an .EXE extension
and the other with a .COM extension, it is very likely
that the second file is infected.
- A file is missing, it cannot be opened, or there is an
unexpected change in the file's properties. Viruses can
delete files or alter them, making them unusable.
- A program closes suddenly for no apparent reason, it
won't run, or your computer stops responding when you try
to use the program. Viruses can alter or delete files associated
with a specific program causing it to no longer function
properly.
- The anti-virus program is disabled and cannot be restarted.
- Strange messages or dialog boxes appear on the screen.
- Other strange effects that can be indications of a virus infection include the CD-ROM tray opening and closing, keyboard and mouse actions taking place automatically, and windows appearing and disappearing at random.