Microsoft has announced that it is releasing another version of XP Professional because it's run out of product keys. The service pack 2c (SP2c) exists for no reason except to provide a way to get new copies of the OS. SP2c doesn't include any new features or functionality.
Microsoft said that SP2c will be released into the system builder channel in September. "Due to the longevity of Windows XP Professional, it has become necessary to produce more product keys for system builders in order to support the continued availability of Windows XP Professional," states the blog entry. Other versions of XP, including Home, Media Center, Pro in the 64-bit configuration, and Tablet PC still have keys available, and aren't part of the new release.