Setup asks for 2 serial numbers

I had ATI 2010 running on a Windows XP system and it works fine.
Now I've bought a new PC, Windows 7 64 bit and tried to install ATI on the new system.
Setup is running to the point where you have to enter your serial number. I copied the serial by CTRL+V. The number was accepted, but a second window with an additional input field occurs telling me that I am trying to overwrite an older version of ATI by a newer one. To continue setup I have to enter the serial number of the older version additionally.
The PC is new and there is no (older) version of ATI installed. So which second serial number do I have to enter?