Unwanted spam e-mails have become arguably the biggest blight of the internet age — and things are getting worse.

One leading UK company reported that spam e-mail doubled in the first fortnight of November, while the e-mail filtering company Postini reports that spam now accounts for 90% of all global e-mail — about 153 billion mails sent every day.

One reason for the rise is that spam has taken a new and threatening twist. Devious spammers are hijacking the home computers of unsuspecting surfers to distribute spam. And if your computer is not properly protected then it’s possible it has already become an unwitting mule, according to Detective Constable Bob Burls of Scotland Yard’s computer crime unit.

This is now a “major threat on the web”, he says. “And instances appear to be on the rise.” Spammers gain control of your computer by infecting it with a trojan, a malicious intruder that masquerades as an innocuous download — a video clip or piece of software, say. Once on your hard disk, the trojan opens your computer’s back door and allows the spammer in.

Spammers have built up vast networks of these “zombie” computers, also known as “botnets”, which run into millions across the globe. Recently Burls and his team uncovered a network of 20,000 “zombies” in London. The internet security specialist CipherTrust claims to detect 250,000 newly infected PCs coming online each day — about 7,500 in Britain alone.

Spam mail may seem harmless, if unwanted, but it contributed to online banking fraud losses of £22.5m in the first six months of this year, according to the Association for Payment Clearing Services. “Phishing” spam — where fraudsters set up fake banking websites, then send e-mails to lure people to enter their credit card details — increased by 1,471% in the past year and was a factor in much of the fraud.

The problem is likely to get worse before it gets better, but there are ways you can repel the zombie hordes and protect your inbox from this junk.

1 WHY DO I RECEIVE SPAM?

Spam derives its name from a 1970 Monty Python sketch set in a restaurant (www.tinyurl.com/ybcbg5) where every available dish is made from the ubiquitous processed meat that became the nation’s staple diet during the second world war. The word “spam” is repeated in a crescendo until the refrain drowns out all other more meaningful conversation — a neat summary of spam’s effect on e-mail.

The first point to recognise is that spam is rarely targeted specifically at you. Instead, various methods are used to acquire your e-mail address from the web. Automated “crawler” programs scour millions of websites to harvest addresses. Spammers then trade these lists. Your best bet is not to publish your e-mail address anywhere online. If you must, then disguise its appearance: for example, spell out in words “yourname at hotmail-dot-com” rather than “ @hotmail.com”, because people can recognise what you mean, whereas spam robots cannot.

Never reply to spam e-mails. A reply confirms to the spammers that your e-mail address is active. Similarly never click on “unsubscribe” links in spam mails. When online shopping and taking out website subscriptions, set up a dedicated e-mail account that is separate from your business or personal address, and ensure you always check boxes to stop your details being passed on to “select third parties”. The fewer times your e-mail address is passed on and the fewer servers it resides on, the fewer chances there are for it to fall into the wrong hands.

Even if you are guarded about your personal address, you may still receive spam. This is because spammers also launch “dictionary attacks” where software guesses at possible combinations of names and e-mail addresses and automatically mails them.

2 HOW DO I STOP SPAM?

Free webmail services such as Hotmail filter out much of the spam before it hits your inbox, but you should tweak the filter settings if your inbox is overflowing. The highest level of protection will receive e-mail only from addresses in your contacts list, and you can also block addresses from specific senders.

If your e-mail software is stored locally, as with Outlook (your mails are kept on your own computer rather than on the web), you can beef up your defences with some carefully chosen security software. You’ll find a trustworthy selection of security applications at www.download.com alongside product reviews from independent experts.

The Download editors give a five-star recommendation to ChoiceMail Free 3.1 (www.tinyurl.com/ymkwwd) with which you establish a “whitelist” of approved contacts. Whitelisting, as opposed to simply using filters, is the single best way to guard against spam. Anybody not on your approved list is challenged when e-mailing you and asked to reply by entering a specific code. This proves a human being is reading the e-mail and not a spamming machine. The mail is then delivered and you can choose whether to add the sender to your whitelist.

CA AntiSpam 2007 (£30 from www.ca.com) offers a more comprehensive alternative, which will build and manage your whitelist and quarantine suspicious e-mails.

3 THE TROJAN THREAT

You might believe you practise safe surfing, but you don’t need to be downloading dubious material to lumber yourself with a trojan. These nasties can hide within all kinds of software, pictures or videos. You should approach with caution sites offering free games, software, pirate music and movies, or avoid them altogether.

A trojan often hijacks control of your computer and turns it into a server that then starts relaying spam. The newest development is that spam-masters do this on a modest scale with each PC, relying on strength in numbers, so that millions of computer owners remain unaware their machines have become “zombies”.

One telltale sign of infection, however, can be a slow-running computer. Try scanning your PC with Spyware Doctor (free from www.pctools.com), which detects and removes many different types of spyware, including most trojans.

4 STAYING SAFE

An unprotected PC is likely to become infected within 20 minutes of being connected to the internet, the SANS web security institute reports (www.sans.org), so an obvious precaution is to equip your new computer with basic defences before plugging into the internet.

If you find it difficult to understand which kind of defensive software is needed, the best bet is an all-in-one protection suite to defend you from almost everything. ZoneAlarm Security Suite 6 (£25 from www.zonelabs.com) is one of the best.

A word of warning, though: one of the most calculating tactics criminals adopt to drop spyware and trojans onto your machine is to set up dummy websites that purport to offer tools for removing their own malicious applications.

Anyone using Microsoft Windows should also make sure all patches are up to date. For more information see tinyurl.com/ydoo9h. Apple Mac users are largely protected from trojans and spyware by the less vulnerable OS X operating system, as are Linux users, but be sure to download every security update when it becomes available.

5 FURTHER INFORMATION

The internet is full of sites offering advice on security issues and some are very good. Get Safe Online (www.getsafeonline.org), was set up by the British government and offers home users many vital tips, including a 10-minute beginner’s checklist of action to take before you go online.

For more on spam, the non-profit group Spamhaus (www.spamhaus.org) offers detailed information on where spam comes from, who the worst offenders are and which countries originate the most spam. For more on the specific kind of spam known as phishing visit the Anti-Phishing Working Group (www.antiphishing.org) or Privacy Rights (www.privacyrights.org/ar/phishing.htm ).

If you want to give spam the heave-ho, report your invaders to SpamCop (www.spamcop.net) by sending it an e-mail through its site. It will then send warning information to the internet service provider responsible for hosting the services used by the spammer.

Will Sturgeon is managing editor of www.silicon.com