Hi Edwin,
It seems to me that the serial number you have is an Upgrade serial number. in such a case the installer check if you have any prior version installed and if it does not find one it prompts to enter the older Acrobat version serial number you owned.
I was able to find Acrobat 8 and 9 registered under your account. Try entering the serial number and check.
Else you need to contact customer service ( Serial number and activation support ) to get the upgrade serial number unlocked.
Regards,
Rave