Microsoft has released a set of tools that will help MSN and Hotmail users block junk email from their accounts.

The move is an indication of the increasing problem of spam, despite Microsoft already blocking 2.4 billion junk emails a day, equivalent to 80 per cent of the messages that hit MSN servers.

In its fight against spam, Microsoft's newest weapon, included in the Hotmail upgrade, stops images in emails loading unless the sender is in the receiver's address book.

This prevents spammers blanketing thousands of addresses with 'web beacon' images in the email that send a message back and indicate whether an account is in use.

"Spam is no longer just an inconvenience for consumers and the online industry; it has become a major problem, one that makes it hard for people to sort through their personal email and reduces productivity," said Lisa Gurry, group product manager for MSN.

Brightmail, whose spam-filtering technology is used in MSN, claims junk email now accounts for nearly a third of internet traffic, compared to just eight per cent two years ago.

Separately, Microsoft announced at the Windows Hardware Engineering Conference (WinHec) in the US that the next version of its desktop operating system, codenamed Longhorn, will be ready in 2005.

According to company executives at WinHec, beta versions of Longhorn will be out next year, with the final version ready sometime in 2005.

The graphical experience and visual quality of Longhorn will be a major feature, according to Microsoft.

Chief executive Steve Ballmer also reaffirmed Microsoft's commitment to digital rights management products in an email to customers this week.

The email revealed no new details but underlined the company's plans for boosting e-commerce in the media sector - which has been embroiled in rows about internet piracy of music, films and games - and for security controls in the corporate environment to protect sensitive data.

"Businesses don't exchange digital information with customers and partners as freely as they might, because they fear it could fall into the wrong hands," said Ballmer.

"These concerns reflect the increasing need of all businesses and many individual computer users to share a wide range of digital information, yet still control who can use it and how."

Microsoft's Rights Management Services product will be available later this year and will also feature in the summer-scheduled Office 2003 suite, Ballmer added.