Bill Gates has reversed Microsoft's previously stated policy with a surprise announcement that a newly developed version of Internet Explorer will begin shipping this summer.

Internet Explorer 7 was due to be released with the forthcoming Longhorn operating system in 2006, but growing concerns over internet security have forced Microsoft to initiate an early release of the code.

The company said last month that XP Service Pack Two would be the last browser upgrade before Longhorn.

"There is only one thing standing in the way of realising the potential of internet infrastructure: the concerns over trustworthy computing," said Gates in his opening keynote at the RSA Conference in San Francisco. "This is the top priority for Microsoft, and it will remain our top priority."

Gates went on to outline some of Microsoft's other forthcoming products. The long awaited antivirus product will be out by the end of the year, and will include multiple antivirus engine scanning and tight integration with email systems.

"The email vector [for viruses] is still key, accounting for 88 per cent of virus incidents," he said. "We need to beef up scanning capabilities there. Having a single engine to do that is not sufficient; we need to take the best ideas of many."

Spyware also come under Gates's microscope. He confirmed that a consumer version of its anti-spyware software, currently in beta, would be available free of charge. But the Microsoft chairman added that there will also be a server edition, for which customers would have to pay.

The beta has so far been downloaded by five million users, half of whom have agreed to be part of the SpyNet network of users who report malicious code to Microsoft's security centre. Gates claimed that the code has so far removed tens of millions of pieces of spyware. The beta can be downloaded here.

On the subject of spam, Gates was in an optimistic mood, claiming that Microsoft's Sender ID system was getting support from email companies to cut spam, and that the company had so far got $100m in potential damages from spammers through the courts.

"We are not at the end of spam," he said. "There is more to be done, but we are past the peak of spam levels."