An unfixed bug in the USB (Universal Serial Bus) driver of Windows XP Service Pack 2 OS causes a notebook's battery to drain faster than usual when there is a device connected to its USB port, Microsoft confirmed Friday.

In a statement through its public relations firm Waggener Edstrom Inc., Microsoft also admitted that the flaw, which lies in the ACPI (Advanced Configuration and Power Interface) driver of Windows XP SP 2, will remain without an easy-to-apply fix indefinitely. The ACPI is part of the OS's power management scheme for USB 2.0, the latest standard for USB peripheral ports on notebook computers.

The flaw affects some Intel-based laptop computers, according to Microsoft. The company informed its support and OEM (original equipment manufacturer) hardware partners of its existence in July 2005 through an article in its knowledge base, a searchable database where support partners can find information about Microsoft technology.

In that article, the company outlined a registry key fix for the bug, but this fix requires a "deep understanding" of the OS and there are risks involved with implementing it so it is not meant for general customer user, Microsoft said.

Microsoft and its third-party support partners are continuing to investigate the bug but do not know if and when they will release a widespread fix, according to Microsoft. However, the company said concerned customers should contact Microsoft technical support and if demand for a patch is sufficient, it will do additional testing and release a fix on its Microsoft.com/download center.

Microsoft said it did not on its own make information about the bug generally available because "the impact was negligible on most systems" at the time the knowledge-base article was released. A published report discovered the bug in late January, and only then did the company publicly acknowledge the flaw.

"While there is a noticeable impact on battery life when USB 2.0 devices are plugged into some specific models of laptop computers, these systems are still fully functional and newer systems have significantly longer battery life than their predecessors, whether a USB 2.0 device is plugged in or not," Microsoft said in its statement.

Also, since behavior of the flaw had not been fully tested when Microsoft released information about it to OEMs, the company said changing the USB driver software might negatively affect how the USB port interacts with other devices. For example, some devices might not work when plugged into the port, or the entire computer might not respond, Microsoft said.