ARIA works as a VST on Windows, yes. Are you asking how to register a Garritan product, or something else?

I purchased GPO 5 about five years ago and have not installed it yet so it’s not the latest update etc.
In the activation instructions it says drop activation code onto the STAND-ALONE version.
I want to use Aria within my DAW as a VST so my question is do I download the standalone version to get activation etc, then download the VST version or can I download the VST version and activate it using that?

A standalone app of the ARIA Player should be installed as part of the complete installation. 

Thanks. Will I have to download both VST and standalone versions?

There are no separate versions to choose. A standalone app and the VST plug-ins are installed as part of the complete installation of a Garritan product.

Thanks for your help.

I have been told that due to the size of the library I should install on a separate drive, have you any advice regarding this please?

It's under 15 Gb, and one of the smallest sample libraries around. You will need to decide if you have room for that on your system drive or not.