Ok first things first -- you can use the same license key from us on multiple accounts and pcs. So if you have a home pc, laptop pc, etc. You can use the same license key (or different ones if you prefer).
Ok now regarding where license keys are stored. On win XP the license key will go in program files directory. And in all cases to AppData. On vista and later, it might be that the license key registered for one user account will have to be put in again if logged in as another user.
More importantly is the question about whether the same configuration directory should be shared by multiple accounts on XP. If you do not want this on XP you will have to edit the configdir.ini file in the program file directory to tell it to use the USERS documents area for saving settings.