You will probably find that your Version 8 is an upgrade and will need a version of the 'full' program installed first. One of your earlier versions will be that full program. Once you find out which it is, that is the only one required to be loaded before upgrading to Ver 8.
For instance, if Ver 5 is the full program, install that, then Ver 8 and skip V6 & V7 etc.
If the above is the case, then I'd be tempted to uninstall the whole Garmin setup and install the 2 needed. Remembering to save any map routes you have in the Garmin folders to somewhere safe.
Hope this helps.