Microsoft has warned customers that they face security risks if they continue to use its older operating systems after it ends support for them next month.

The systems affected are Windows 98, Windows 98 Second Edition and Windows Millennium Edition. Support will end on July 11, the next date for Microsoft's monthly security patches and software updates.

Security updates will also end, a posting on the Microsoft Security Response Center said.

Microsoft said it wasn't feasible to make extensive changes to eliminate a security vulnerability since the underlying architecture of Windows Explorer for older versions of Windows is much less robust, wrote Christopher Budd, a program manager with Microsoft's Security Response Center.

"Due to these fundamental differences, these changes would require reengineering a significant amount of a critical core component of the operating system," Budd said.

As a result, applications may not run on the updated system, he said.

Microsoft advised those still using the operating systems put them behind a perimeter firewall that filters traffic on TCP port 139, which will block attempts to exploit the problem, Budd said.

Further, support for Windows XP Service Pack 1 will end on October 10.