I just need to backup the installer right? Not install the game and back that up?

Just download the installer (setup*.exe) and any patch files if there are newer patches, along with the DLC (and any patches), and if you want to backup all of the bonus goodies etc. as well. You can download them all either using Galaxy, or using the GOG Downloader or direct with a web browser. On rare occasions a download link might not work or generate a server error in Galaxy, but yelling at Judas a bit, or filing a bug in mantis usually gets it fixed within 24-48hrs. :)

Also there is a script someone wrote called gogrepo.py which you can find if you search the forums here. It can download backups of your entire library and keep them updated over time etc. however I haven't used it personally yet.

Hope this helps.

Typically you want to look over your shoulder before doing either.

*flees*