A virus is a program, which when loaded onto your computer system, will execute itself and cause damage to the system. The virus can spread itself to other computer systems.

A worm is a program that is able to replicate itself and disrupts the computer system by making so many copies of itself that it uses up all the system resources. It too can spread itself to other computer systems.

A Trojan horse is a malicious program that is disguised as something harmless. Unlike viruses and worms, a Trojan horse cannot replicate itself however it may spread as part of a virus.

The best way is to install anti-virus software on your computer and update it regularly. The anti-virus should be configured to constantly scan your computer every time it is switched on. It should also scan all email attachments and files downloaded to the computer be it from a website, instant messenger or any other programs.

Popular and highly-rated anti-virus software:

• Norton Antivirus
• McAfee VirusScan
• PC-cillin Internet Security

For anti-virus reviews, check out CNET's Security & Utilities Reviews.

Scan your PC with your anti-virus program to detect any possible virus and worm infections. If your anti-virus software is active and you receive a virus-infected email, the anti-virus should detect it and alert you.

Alternatively, you can try online virus scans such as

• Symantec Security Check
• HouseCall
• Command on Demand

You should stay offline as much as possible because the virus will spread whenever you are online. If your anti-virus cannot remove the virus, contact the vendor for further assistance. You may also refer to the anti-virus vendor's website - some anti-virus vendors provide special removal tools for a particular virus.

Get the full headers of the virus mail and send it to the person's ISP. Ensure that you do not send the virus attachments because the email will most probably be blocked by the ISP's anti-virus.

If the sender was a SingNet subscriber, you may report it online at the Internet Abuse Incident Report.

There are some possible scenarios that could explain this:

• The From field was forged by the virus.
• Some email clients store all email addresses found in the emails received. So if you and the virus-sender share a mutual acquaintance, then your email address could have been stored when your friend sent an email to both of you.

There are legal restrictions preventing the release of a subscriber's particulars unless pursuant to an official request by the police or a Court Order, etc. If you feel that you have grounds for legal action against the spammer, please consult your lawyer for further advice.

Most viruses are able to send out infected emails from the infected computer system without the user's knowledge. The virus starts sends the infected emails when you are online and the emails are not listed in your Outbox or Sent Items folder.

In this instance, changing your email address will not solve this problem as it is your computer that is infected with the virus. You should take immediate steps to remove the virus from your computer system and try to remain offline until you are certain that the virus has been cleared.

Again, there are several possible scenarios:

• The recipient's email address may be in other files on your computer - some virus strains are not limited to the address book in the search for email addresses
• The virus may be programmed to send emails to specific addresses
• The virus may generate email addresses e.g. insert the same UserID but append a different domain each time e.g. The UserID 'admin' is used with the recipient's domain name e.g. admin@singnet.com.sg.