There are several ways a computer virus can enter your computer.
1. Through e-mail attachments: Never open attachments to e-mails coming from unknown sources. They are probably number one source for computer virus. In fact, don't even open e-mails coming from unknown or dubious sources. When in doubt, immediately delete the e-mail along with its attachment.
2. Through downloaded files from the web: Download only from well known and reputable sites. Try not to download anything from unknown, mysterious sites. In any case, never open any downloaded file before scanning it with an anti-virus program.
3. Through floppy disks: Floppy disks that are used in many computers, such as office computers, are notorious for carrying viruses and infecting many computers. If you have to insert such a disk in your computer's floppy drive, first scan it with an anti-virus program before opening any file. If you ever inherit any old floppy disks from anybody, before reusing them first format them. That way, any virus present in the disk will be destroyed.
Your best protection against virus is to install a good anti-virus program in your computer. These programs can be scheduled to scan your computer for viruses automatically during start-up, at closing time, whenever you download a file, and so forth. The program runs in the background the whole time your computer is on, and it automatically starts scanning whenever something suspicious happens, e.g., when you are surfing the Net.
But the key is to keep the virus list (often called virus definitions) upto date. New viruses and their antidotes are being discovered almost everyday. So with an out dated virus list, your anti-virus program may not give you adequate protection against recently introduced viruses. Almost all good anti-virus programs will let you connect to their site either through the Internet or via modem, and download the latest virus definitions. You should do this at least once a month and 2 or 3 days after you hear on TV and radio that a new virus is creating havoc. I do it 2 or 3 days after hearing about the new virus, because I know by then the leading virus research laboratories would come up with an antidote for that virus. You just have to pay a nominal annual fee to your anti-virus company to be able to connect to their "Live Update" program and download the latest virus definitions. Considering the protection you get, the fee is worth paying.
The above paragraph was written around July of 2002. Since then I have upgraded my anti-virus software to Norton 2003. This automatically updates its virus definitions as soon as I connect to the Internet if an updated definition is available, and then it notifies me that my virus definitions have been updated! What could be more convenient than this?
A note about anti-virus programs: These programs must be turned off during software installation. Otherwise, after the installation, your computer will cause problems. You will get error messages, your computer will malfunction, it will freeze, and so forth.