If you click the blue button on the Kobo website that is labelled "ADOBE DRM EPUB" it will download an ACSM file, that when opened will invoke ADE that in turn will download the ePub and store it within the "My Digital Editions" directory.
In turn, you can then attach your Kobo and copy that book over, it will be on the Touch.
Conversely, once you have bought the book, Kobo Desktop will download a Kobo ePub format to your PC and store it in your %APPDATA%/../local/Kobo/Kobo Desktop Edition/kepub but it is encumbered by Kobo's own DRM.
The next time you either attach the Touch via USB to your PC that book will sync over, or the next time you connect via WiFi, that book will also sync over.
The only way to prevent this is to go to "My Library" on the Kobo books web site
http://www.kobobooks.com/library/reading.html and "delete" the book from that section. It will stay within your "Trash" of your library, and can be restored from there.