SEATTLE (Reuters) - Microsoft Corp. MSFT.O said on Thursday that a security flaw in all versions of its flagship Windows operating system software released since Windows 98 could allow attackers to delete digital certificates.

The world's top software maker said that an infiltrator could use an e-mail or a Web site to gain entry into a system and delete the pieces of data that are used to encrypt other data, such as e-mail messages.

Microsoft labeled the security flaw as "critical" and said a patch was available on its technical Web site (http://www.microsoft.com/technet). Microsoft said that its Internet Explorer software could be configured to defend against an attack.

The versions of Windows that are vulnerable include Microsoft Windows 98, Windows 98 Second Edition, Windows Millennium, Windows NT 4.0, Windows 2000 and Windows XP.

Microsoft, shaken by break-ins to its system and vulnerabilities in its software, launched a "trustworthy computing" campaign earlier this year to improve the security of all its software.