Yes the serial number can only be used once. After that you go to your online account to download the software.
Antivirus Software and Internet Security For Your PC or Mac | McAfee (Change the country at top of page if necessary).
Run the MCPR cleanup tool before trying to reinstall. That will clean up any remnants of the old software. Reboot and then try your installation.
If it continues to give trouble then contact Technical Support, it's free of charge
Moved to the General Discussion area as a better spot
Thank you very much for your reply !
These were exactly the steps I followed. But every time I entered a new serial number the system gave me the error message.
I think that it was a bug on McAfee server, because I tried again this morning and it worked perfectly.
Thank you anyways for your quick reply !
Glad you are OK.