Can't Install Paid Version

I downloaded a trial version of Parallels which expired ages ago, I un-installed it, and then just recently bought the software, I installed it and it keeps saying the software has expired, and gives me no where to enter my serial number, I have uninstalled using the un-installer however when I re-installed I notice that my verison of Windows was all still in tack... What is the best and safest way to get rid of everything to start again? Also the box says once I install the disc version to then download a new build and upgrade, is there any point in installing the Disc version can you just install Build 5608?

 

Please download build 5608 following link bellow:
http://download.parallels.com/v3/en/GA/Parallels-Desktop-5608-Mac-en.dmg

and reinstall Parallels Desktop as described in kb
http://kb.parallels.com/en/4790