Project Dll Inclusion

I know that part of this topic has been answered in the forums, but I'm still not being able to make it work:

I'm using System.Data.dll in my project, since I need to use SQL Server.

When I run in the editor, everything works just fine, but when I "Build" (for PC, Windows, x86), and run the .Exe, I get a message in the scripting console saying "NotSupportedException: Codepage 1252 Not Supported".

So, after googling I found that when this happens, it's necessary to place the "System.Data.dll" file, from "C:\Program Files\Unity\Editor\Data\Mono\lib\mono\unity" into the "Plugins" folder (Assets -> Standard Assets -> Plugins - I think this is the correct location).
So, now the problem is that after placing the file in that location, I get an error in Unity saying:
"Assets/Standard Assets/Scripts/User Scripts/Register/Script_Register.cs(69,33): error CS0433: The imported type 'System.Data.SqlClient.SqlConnection' is defined multiple times.

How can I solve this situation and use SQL Server without all theses errors?

My thanks in advanced

 

Ok, sorry, there was another System.Data.dll in a project folder, already.