Microsoft has quietly fixed a problem with Windows XP that disables the power management functions of AMD's line of mobile processors. The fix allows users of laptops based on Athlon 4 and Duron mobile chips to use PowerNow! technology, which extends battery life by reducing processor power when it isn't needed by applications. The glitch affects users who upgrade AMD notebooks to Windows XP from an earlier version of the OS. The version of XP available on retail shelves doesn't include a driver -- amdk7.sys -- needed for PowerNow! to function, although the driver is included with new AMD laptops running Windows XP, according to AMD. The driver posted on Microsoft's site is dated from late October, only days before Microsoft released Windows XP into the retail channel. The incompatibility has not been widely publicised, however, and may be a blow for AMD's hardware platform, which is advertised as delivering "outstanding performance with Windows XP". The newer desktop processors have even been named Athlon XP, partly as a way of emphasising their Windows-friendliness. AMD product manager Dave Everitt admitted the problem will be an inconvenience to users, but said installing the driver is "a very easy upgrade". A statement on Microsoft's support Web site reads: "Windows XP Home Edition and Windows XP Professional do not have the ability to take advantage of the AMD PowerNow! functionality. Therefore, laptop computers with the AMD Athlon 4 processor do not have the improved battery performance that is offered by the PowerNow! technology." The driver can be installed via Microsoft's Windows Update Web site, which will automate the process, or downloaded directly from Microsoft's support site. However, manual installation of the driver involves editing the registry, which can disable the operating system if done improperly, so Microsoft doesn't advise it. "If you are not severely affected by this problem, Microsoft recommends that you wait for the next Windows XP service pack that contains this fix," says a statement on the support site. Microsoft hasn't yet released a service pack containing the fix. On Monday, AMD ramped up its mobile processor line, launching a 1.2GHz Athlon 4 and a 950MHz Duron. In tough market conditions, AMD is betting on success in the mobile and server areas to increase its market share. See Chips Central for the latest headlines on processors and semiconductors. To find out more about the computers and hardware that these chips are being used in, see ZDNet's Hardware News Section. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the Chips Central forum Let the editors know what you think in the Mailroom. And read other letters.