I'm using Windows, and I want to use Dropbox to back up a folder outside my Dropbox directory

The equivalent of a symbolic link on Windows is the junction point. You can create junctions with the appropriately-named utility Junction from Mark Russinovich at Microsoft.

junction.exe C:\Users\Zachary\Dropbox\Documents C:\Users\Zachary\Documents

On Windows Vista and later, there also exists the command line utility mklink.