Trojans now outnumber viruses and worms four to one

Sophos security said that the 10 most commonly found pieces of malicious software all targeted Windows machines.

In contrast, it said, none of the "malware" were capable of infecting the Mac OS X operating system.

Microsoft has pledged that the latest version of its operating system, known as Vista, will be its most secure yet.

One analyst told the BBC News website that the reason why there are fewer viruses and worms on Mac systems is that the smaller user base makes them a less attractive target for hackers.

Microsoft Windows currently runs on nearly one billion computers worldwide. The latest version will be released later this year.

"It is our goal to give PC users the control and confidence they need so they can continue to get the most out of their PCs," a Microsoft spokesperson said.

"Windows Vista contains a number of new safety features that, taken together, are designed to make Windows PCs more secure and online experiences safer."

Microsoft said that security on Vista would be an integral part of the operating system rather than an add-on like in previous systems.

Top threats

The advice from Sophos was given as it released a report, detailing the security threats posed to computers so far in 2006.

The report says that there has been a vast drop in malicious software like viruses and worms.

It seems likely that Macs will continue to be the safer place for computer users for some time to come

Graham Cluley, Sophos

Trojans are pieces of malicious software that are hidden in other legitimate programs such as downloaded screensavers.

The Trojan may collect financial information or allow the infected computer to be controlled remotely for sending spam or launching web attacks.

"The continuing rise of malware will concern many - the criminals responsible are obviously making money from their code, otherwise they'd give up the game," said Graham Cluley, senior technology consultant at Sophos.

Mac flaws

Although Trojans dominate the list of security threats, the most widespread problem was the Sober-Z worm.

The worm, which was spread by e-mail, infected people's computers and tried to turn off security settings. It replicated by looking for other e-mail addresses on the computers' hard drives.

STAYING SAFE ONLINE Install anti-virus software Keep your anti-virus software up to date Install a personal firewall Use Windows updates to patch security holes Use reputable anti-spyware programs such as AdAware or Spybot Do not open e-mail messages that look suspicious Do not click on e-mail attachments you were not expecting

The worm infected computers running the Windows operating system, but was not designed to infect Apple Macs.

"It seems likely that Macs will continue to be the safer place for computer users for some time to come," said Mr Cluley.

"[That is] something that home users may wish to consider if they're deliberating about the next computer they should purchase," he added.

But Brian Gammage, an analyst at Gartner research does not believe the advice is well founded.

The number of people using Macs is far less than those using Window's based PCs, he said.

"If you have smaller walls, you attract less graffiti," he said. "There is nothing architecturally safer about Macs. If everyone moved to them then the situation would change overnight"

Earlier this year, a security flaw in the way that Macs downloaded files was identified; while three concept viruses and a worm written specifically for Apple computers were also discovered.

The viruses were never released into the "wild" and posed little security threat.