The company had previously said the free update, called Service Pack 2, would be available sometime this summer but did not offer specific date.

Mr Mike Nash, Microsoft's corporate vice-president in charge of security, said on Monday the company still couldn't say exactly what day Service Pack 2 will be available, but that they do expect it next month.

The release was once promised as early as June, but has been delayed by last-minute changes - including making sure new security precautions don't cause other applications to stop working.

'In the trade-off between security and convenience, we want to define the right balance there,' he said.

Microsoft, dogged for years by attacks that take advantage of vulnerabilities in its dominant Windows operating system, is hoping the upgrade will reduce disruptive and costly interruptions.

For example, the new system will automatically turn on Windows Firewall, which aims to guard consumers against unsolicited downloads that could track keystrokes, steal information or otherwise act maliciously. The company's free e-mail program, Outlook Express, also will have stronger default protections against downloads that could be used to spread viruses