SQL Connection Login

I'm developing an app that when it starts, ask the user the username and password, if they are logins users in SQL 2000 they must enter the system, but i'm not really sure where can i declare that part?

i mean, this is my SQL connection string at the moment:

SqlConnection con = new SqlConnection("Server=VICPC;Database=northwind;UID=sa;PWD=admin;");

......

con.Open();

......

con.Close();

but that's the default SA login for SQL, what i need to do is to aprove that an user account actually exists in the system so i can see wich tables show him, and the restrictions to apply to him, if i just change this:

SqlConnection con = new SqlConnection("Server=VICPC;Database=northwind;UID=victor;PWD=password;");

it shows me just what i configured in the SQL account for that user?, the tables in the database Northwind and all that? or i'm missing something else? lets say that the user "victor" is not allowed to delete any records from table Categories, if i use that connection string it blocks me from deleting records as it should?

By the way the code example here is C# and this question is more related to SQL than C#, i hope you don't mind about the language, but i needed a piece of code to show :)

I'd appreciate any help

Thanks