It takes just a few seconds. Open Windows Explorer (Start -> Computer, or Windows key + E), and click the address bar. Copy and paste the following address into it:
%APPDATA%\Microsoft\Windows\SendTo
Here, drag and drop the Dropbox shortcut from your favorite folders:
If you don't have it in your favorites, you can bring it from whatever directory it is in. If you drag it in from the faves, and it disappears from there, you can drag it back in, and it works perfectly. You can rename the shortcut you just created, and it will appear something like this after everything's done:
For instructions on how you can do it in XP, go here.