|Having a good virus checker is a wise
precaution, whether you are on the Net or not. However, note that viruses
infect your system when you run a program (which may have the extension
.com or .exe but there are other possibilities)
or a macro (in Word or the like) - they don't hide inside ordinary text
files (see also section 5.8). Be suspicious of files with more than
one extension (e.g. report.txt.exe).
Exercise caution (and your virus checker) when running programs from sources that you can't be certain are reliable, such as WWW "home" pages. Also beware of emails containing attachments that you are not expecting. Even if you know the sender it could be that their computer has become infected. Note that Microsoft do not send out security patches by email and any email claiming to contain such a patch almost certainly has a virus attached.
A WWW search for Virus Information will produce a mine of detail if you need to know more - e.g. the Computer Virus Information and Resources Page at http://csrc.nist.gov/virus/
Companies producing anti-virus software include:
Remember that the software will need to keep abreast of the latest viruses in circulation. To do this you will need to set your anti-virus program to check for the updates that are issued very frequently (often weekly or even daily) by its manufacturer.