This section of the PID describes the channel (OEM, Volume License, Retail, bundle/Not For Resale) a given XP CD/system has come from. Other product IDs with the same format, such as those found in Microsoft Office (and many other Microsoft products), serve the same purpose. The xxxxx section is the Microsoft Product Code, which in this case describes the platform, build, and version of Windows. You only have to worry about your platform (currently, whether you are working with 32-bit or 64-bit platforms) since each platform is on its own disc. There is no need to worry about OEM/Retail/Upgrade issues when working with Vista. Starting with Vista, all product SKUs are on one disc.
