The patch, according to Microsoft's Web site, resolves a problem affecting notebooks based on Intel's Pentium III processor and 440MX chipset and running the Windows XP operating system. The laptops appear to hang when their internal modem is put into use.
Notebook users whose machines exhibit the problem can download the patch via Microsoft's Windows Update Web site. An update to the machine's BIOS (Basic Input Output System) software--special software that loads the operating system when you boot up your PC--may also be required on certain machines.
The issue is somewhat limited, affecting only machines with the 440MX chipset. The chipset is used widely by PC makers, but only in a specific class of machines known as "ultraportables." Ultraportables generally weigh less than 4 pounds. The class represents a relatively small portion of the notebook market as a whole, about 10 percent to 15 percent.
The problem has not been seen with other versions of Windows.