It apparently denotes if this is the first time a user is running Explorer - I've seen it many times on Vista and Windows 7 machines, but never on XP. My guess is it's part of shell first run on Vista and higher, an educated guess, but with no real supporting evidence. I've thought about setting procmon to run a boot trace, create a new admin user, and then reboot and watch for the existence of, or creation or modification on, that registry location, but I've never bothered. Might be a good idea though if you're really curious.