The software giant, which has been repeatedly blamed for leaving security loopholes in its software, said that it could not eliminate threats similar to Sobig without a complete redesign of the personal computer — an effort that would not conclude until “2005 or 2006”.

Detlef Eckert, Microsoft Europe’s senior director responsible for the security of its software, said: “A virus like Sobig exploits user behaviour to help it spread. Even with the best software you can design today, an attack like Sobig is always possible.”

Sobig lurks in the guise of an e-mail attachment which typically appears to have been sent from somebody known to the recipient. If the attachment is clicked on, the virus runs a program that sends copies of itself to every e-mail address it can find, generating a high volume of traffic that can clog up computer networks. Last week, the latest version of the virus, Sobig.F, generated so many messages that it clogged up the internet.

The computer virus is technically difficult to prevent because of the inherent difficulty in stopping computer users from inadvertently running attachments from what appears to be a plausible or attractive source.

Mr Eckert added: “We’ve already seen many viruses that work in a similar way to Sobig. The Kornikova virus, for example, spread (in 2001) because it promised to show users a picture of Anna Kornikova (a female tennis player) in her natural form.”

Currently the only option available for a company, or internet provider, is to install software on its link to the internet that blocks out all “executable” attachments — attachments that run a computer program. However, that solution is crude, and could lead to large numbers of attachments being blocked. Many organisations do not deploy such a block.

Microsoft’s plan is to introduce a special “safe” environment as part of a wider effort to develop what it calls the “next generation secure computing base”. Mr Eckert added: “We aim to create an environment in which you can run unknown code securely, and so preventing problems for users who, in a moment of weakness, have clicked on the wrong button.”

The company is working on the initiative with hardware vendors led by the chipmaker Intel and the computer manufacturers IBM and Hewlett-Packard. More details are due to be unveiled in November at a software developers conference in Los Angeles.

Despite problems in dealing with Sobig, and viruses like it, Microsoft insisted that it had increased its investment in improving security. It has set up a programme, called Trusted Computing, which last year led to the training of 8,500 company developers at a cost of $200 million (£127 million).