The Internet has been challenged again as a secure medium amid reports this week of a serious defect in several of the most popular home and business email programs.

The flaw could allow someone to use the Internet to access and pirate another computer's files, erase the hard-drive or crash a computer system.

The flaw, which exists in Microsoft's Outlook Express and Outlook 98 and Netscape's email program in its Internet suite Communicator 4.0 and 4.5, allows a virus to be sent into a computer through an attached file. Outlook Express is the email program included in Microsoft's browser, Internet Explorer 4.0 or 4.1, which ships with Windows 95 and Windows 98. Microsoft said the flaw is also present in Outlook Express for Windows NT 4.0, Windows NT for DEC Alpha and for Macintosh and UNIX machines. Users would need to open the attached file before the virus would be executed, but an additional worry for Outlook 98 users exists because the program automatically opens attachments when an email message is received.

Netscape said the flaw affects Windows and Windows NT versions of Communicator but not Macintosh or UNIX versions. Both Microsoft (www.microsoft.com) and Netscape (www.netscape.com) are posting patches (small programs which correct a program glitch) for the problem, but Microsoft warned that its initial patch, posted on Monday, was ineffective and users would need to download a newer version.

The flaw was first discovered by a Finnish research team last month and was initially reported on Tuesday by the San Jose Mercury-News in California. The flaw has caused widespread concern; a US government department even issued a security alert in the wake of the flaw's discovery.