Several photographers have asked about running Lightroom 2 as a 64-bit application on Windows XP x64. Lightroom will not install as a 64-bit application on Windows XP by default because it's not an officially supported platform for our 64-bit version. (Lightroom is officially supported on Vista x64)
But when you run the Lightroom installer and choose to "Open the Specified Folder" in the Files are Ready dialog (Be sure to uncheck the option to "Launch the Adobe Photoshop Lightroom 2.0 Installer") you'll see an Adobe Photoshop Lightroom directory that contains "setup32" and "setup64" executables. I'll let you pick the one you think most appropriate.