You will need to install Adobe Digital Editions (ADE)
http://www.adobe.com/products/digitaleditions
Run ADE and authorize your computer.
Then sign into your Kobo account on the kobobooks.com website, and click on LIBRARY (top right hand of the blue header bar). For each book, click on Adobe DRM ePub (most books have this blue button). This will allow you to open or download an .acsm file. This is not a book, just a link to a book. Open each acsm file, which will download the ePub, and save it in the ADE library.
When you are done, click on the library view in ADE (top left hand icon). hover over the title of a book. There is a right-arrow; click it and a dropdown will appear where you can see "Item Info". Hover over the Filepath and it will display the location of the file. On my computer it's C:\Users\<username>\Documents\My Digital Editions.
In that folder, you should find all your ePubs, ready to archive. These books are DRM-protected and tied to your Adobe account.
(One of my books does not have an ePub. It is a public domain, free book)