Hi Nick,
You have the April 2006 service pack install - not the March 2006 upgrade install. The big difference is that a service pack cannot be installed on top of a different version. As you indicated, you are running version 8.0A - the reason you are getting the message you are.
You first need to download and install the upgrade version for 8.0B before service packs can be put in. You would have been sent an email back in March with the link to the download, as long as your Upgrade support plan was current. Once that is done, then you will be able to install the service pack.
If you don't have that email anymore, please contact
info@softrak.com for them to re-send the upgrade email to you.