Neil
The serial number is typically on the box housing the envelopes with the installation discs (one for Windows and one for Mac).
Are you entering the serial number with the -
xxxx-xxxx-xxxx-xxxx-xxx-xxxx
Disable the antivirus and firewall(s) before you begin.
Please give this another try and then we will decide what next which may include using some alternate installation
files into which to insert your purchased serial number.
Thanks.
ATR