Here is a nice tutorial that you can use to run multiple instances of Dropbox on your Windows OS. The tutorial is split into two section. The first part shows you how you can use multiple instances of Dropbox on Windows 7 and Vista and the second one shows how you can do the same in other versions of Windows.
Run Multiple Instances of Dropbox on Windows 7 and Vista
1. You will need to have atleast two user accounts on your Windows 7 or Vista. If you do not have one already, please create one. The new user account does not need to be an administrator account (though it is convenient to make it an administrator account as Dropbox installation file may be blocked using group policy) but it should be password protected. The password you choose can be a simple one. Let me create a new user account named alter.
2. Now install Dropbox and configure it on your primary account which in my case is Abbu. Use any one of the Dropbox account you are using. If you have it already configured, skip this step.
3. Now log off from your primary Windows user account and log in to the other one which is alter for me. Once you are logged in, install Dropbox and configure it with the second Dropbox account. Make sure the Dropbox folders for the two Dropbox accounts are not same. Log out of the user account.
4. Now log in to your primary user account once again. Open windows explorer and go to
[OS Drive Letter]:\Users\2ND_USERNAME\AppData\Roaming\Dropbox\bin\
5. You will find the Dropbox.exe file there. Right-click on it and choose Send To > Desktop (create shortcut).
6. Now as the shortcut appears on desktop right click on it and select Properties. As the Properties dialog opens go to the tab Shortcut.
7. Now in the Target field append the command runas /user:2ND_USERNAME before the filepath. So for me the string looks like,
runas /user:alter C:\Users\alter\AppData\Roaming\Dropbox\bin\Dropbox.exe
8. Now click Apply and then OK to save the changes. Double-click the newly created shortcut to start the second instance of Dropbox. You will be asked to enter the password for the secondary user account. Type the password and press Enter.
9. Take a look at your Windows notification area and you will see two animated Dropbox icons. You can also open task manager to view two instances running.
This way you can you as many accounts as you choose but for every account you will need a different User Account on your Windows OS.
Run Multiple Instances of Dropbox on Other Versions of Windows
For all other versions of Windows you can use Dropboxen, a Dropbox addon. To run multiple instances of Dropbox just download the addon from the Dropbox website and follow the tutorials there. You can also set up a Dropbox uploader where your friends can upload files to your Dropbox account. If you face any problems setting it up yourself feel free to ask.
[via Semi Legitimate]