: Refers to Windows XP Professional x64 Edition , a 64-bit operating system based on the Windows Server 2003 codebase.
The file is a compressed archive typically associated with community-maintained updates, drivers, or specialized software for the 64-bit version of Windows XP (Windows XP Professional x64 Edition). File Overview XPx64.7z
While the specific contents depend on where you downloaded it, files with this naming convention often contain: : Refers to Windows XP Professional x64 Edition