Worms
These viruses spread via computer networks. The ILOVEYOU virus above was a classic example of a worm. These viruses are becoming an increasing threat as a growing number of computers are permanently connected to networks. Worms can spread over corporate networks or via emails sent over the Internet.

Trojans
A Trojan virus takes its name from a story in Homer's Iliad where Greek soldiers pretended to make peace with their enemies, the Trojans. The Greeks made a grand peace-keeping gesture - the gift of a large wooden horse.
When the Trojans hauled it inside their city gates, a small band of Greek warriors leapt out. They opened the gates and let the rest of the Greek army storm in to capture the city.
A Trojan virus is one that opens your computer up to malicious intruders, allowing them to read your files.

File viruses
A file virus is one that replaces a key system file on your computer. These viruses can reload themselves every time you start your computer up. Once they're in the memory, they can spread by writing themselves to any disk you insert into your disk drive.

Boot sector viruses
This is an early type of computer virus that spreads by hiding itself in an invisible location on your hard drive or floppy disk. When your computer reads an infected floppy disk, the virus is copied from the disk to your computer's memory.
From there, it writes itself to the 'boot sector' on your hard drive. The boot sector is read each time you turn your computer on. So the virus is constantly reloaded and can copy itself on to other floppy disks. These viruses are fairly rare nowadays, as they are easy to catch.

Macroviruses
A macrovirus infects word processor files, such as Microsoft Word documents. Although not as dangerous as other viruses, they can spread quickly if a Word file is sent via email. After an initial scare, Microsoft added protection into later versions of Word, so you receive a warning about infected documents.

Hoaxes
The virus hoax came about after friends sent each other emails about a new virus threat. Someone decided that they could cause just as much trouble by sending out fake warnings rather than real viruses.
Hoaxes may seem harmless, but they do a great deal of damage to the Internet as a whole. Not only do they slow down traffic and clog up email servers, but they also cause people to panic. Companies can spend money and time investigating what is just someone's idea of a joke.

How do they do that?
Before you go and spend hard earned cash on a virus scanner it is worth understanding what they do.

A typical virus scanner will have two main functions:

• scan for new viruses entering your computer
• clean up any viruses it finds and make sure they can't do any more harm

Virus scanners are pieces of software that run all the time. So when you start up your computer the virus scanner will also start. This ensures that viruses are caught as soon as possible. A virus scanner will also check the following for viruses:

• disks inserted in your PC
• email you receive
• programs you download from the Internet

If you have already got a computer virus, the software will find these and:

• Notify you that you have a virus (stay calm!)
• Try to repair the file the virus has infected
• Isolate any files that can't be repaired

The virus scanner will always try to rescue any files it can. There are some viruses that don't destroy files, instead they 'corrupt' them so they cannot be used. Good virus scanners can often reverse this.

If the scanner cannot repair a file, it will place it in a secure area so you can't run it by accident. Then you can send the virus to an anti-virus company. If it is a new one they will add it to their database and at least you will be able to help prevent other people catching it.