The following information was provided by Carey Burgess (Adobe Employee):

Does Lightroom 3 launch and work fine in an admin account, but when you launch the application in a standard (i.e. non-admin) account it asks for a serial number?

If so, then it is likely due to a file permissions issue either in the original (admin) account or with the standard account.

(Although you need administrator access to install Lightroom, and you do need proper access permissions to your photos, you should be able to run Lightroom without issue in a standard user account.)

Lightroom 3 stores registration data (the serial number) in a file called Lightroom 3.0 Registration, which is stored by default in one of these locations:

    * Mac OS: /Library/Application Support/Adobe/Lightroom

    * Windows XP: C:\Documents and Settings\All Users\Application Data\Adobe\Lightroom

    * Windows 7/Vista: C:\ProgramData\Adobe\Lightroom

-  Check for the registration file in the appropriate location for your system.

If the file does not exist in that location, then ensure you are logged in with the admin account (the one where Lightroom doesn't prompt for a serial number) and check the user-specific location instead:

    * Mac OS: /Users/YOUR USER NAME/Library/Application Support/Adobe/Lightroom

    * Windows XP: C:\Documents and Settings\YOUR USER NAME\Application Data\Adobe\Lightroom

    * Windows 7/Vista: C:\Users\YOUR USER NAME\AppData\Roaming\Adobe\Lightroom

If the file exists in the user-specific location instead, then I would suggest that you ensure that you have quit Lightroom, and then move the registration file from the user location to the default system-wide location. Lightroom should now no longer prompt for serial number when you open it from a non-admin account.

If the registration file did (or now does) exist in the default system-wide location, but you are still being prompted to enter a serial number when opening Lightroom in a standard user account, then that account does not have sufficient access permission to the registration file.

To resolve this aspect, there are two possible solutions (the first being the recommended solution):

1. Change the access permissions on the Lightroom folder that contains the registration file to grant "Read" access for the standard user account. (You can do this for each account individually, or you can use a broader group like Everyone.)

For more information about changing permissions, see one of these articles:

    * Windows XP: http://support.microsoft.com/kb/308419#4

    * Windows Vista/Windows 7: http://windows.microsoft.com/en-us/windows7/What-are-permissions
    * Mac OS: http://docs.info.apple.com/article.html?path=Mac/10.6/en/8342.html

2. Copy the registration file from the system-wide location to the user-specific location for the standard user account. (You would need to repeat this step for each standard user that needs access to Lightroom.)

Reply here if these steps do not resolve your issue, or if you have any questions.