Support for the operating system officially ends next month on July 12.

The vulnerability exists in Windows Explorer and the way it handles Component Object Model objects, whereby a malicious Web site could force a connection to a remote server where Explorer could fail, executing arbitrary code and giving the attacker complete control of the OS.

Patches correcting the flaw were issued for Windows 2000, XP and Windows Server 2003, but the vulnerability remains unpatched on Windows 98.

Christopher Budd, Microsoft Response Center security program manager, said the upgrades to Explorer's architecture since Windows 2000 has left 98 behind and applying fundamental changes could jeopardize program compatibility.

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