How to install Dropbox in Windows 7 for all user accounts?

Dropbox offers an AddOn called DropBoxen that can accomplish what you are trying to do:

Dropboxen enables you to run multiple Dropbox clients concurrently by automatically launching multiple instances of Dropbox.exe as different Windows users so Dropbox will store the shared files in each unique Windows user directory.

It's not ideal and there are some specific problems and workarounds with trying to use this on Windows 7, but if you really want to accomplish this, its possible.