Consumers are being warned not to fall for a spam campaign claiming to offer a cut-price edition of Microsoft's new operating system, Windows Vista.

The spammers are using new techniques to bypass spam filters to offer recepients a bargain edition of Vista . The spam email claims that the recipient can save $319.05 (£162) by downloading Windows Vista 'today'.

One thing that is not quite clear is what anyone who falls for this scam will actually get.

According to security company Sophos, people could be paying out for a pirated copy of Vista and find themselves with a whole host of problems, or it could be a phishing attack which is being used to steal credit card details.

However, the spam is a clear indication of how fraudsters are testing computer security to the limit. The technique used in this case is what is called image spam. This is when a graphic is embedded in an email rather than regular text.

It has grown in popularity amongst spammers attempting to communicate their marketing messages to internet users. Often image spam is used for promoting stock pump-and-dump scams or drugs to help with weight loss and sexual performance.

Now the latest widespread campaign said Sophos is concentrating on this 'bargain' edition of Vista.

"This widespread spam campaign carries all the hallmarks of a typical image spam. The spammer has added speckled noise in the form of random pixels to make the graphic slightly different on each sending, and users are told to type in the name of the website rather than clicking on a link," said Graham Cluley, senior technology consultant for Sophos.

"Approximately 30 per cent of all spam now uses images to try and sneak past anti-spam filters. Computer users need to ensure that they have strong defences in place or they will continue to be bombarded by nuisances like this."

He added: "It's worrying just how poorly educated people are about web and email threats. The average man in the street hasn't got a clue."