How To Know Existence Of The Data In The Databse

Hi Friends,
I have one table in the databse,i.e userTable with one field userName
In my form I have one Label ,textbox for entering the userName and One button for submit,
So I am entering the data into the table(userTable) after clicking on the submit button
But my problem is before entering the data into the table I want to find wheather the given data exits or not
if its not exists the data has to insert into the table otherwise its has to display the message"the user is already existed"
for this I wrote the code like this in C#
 
public Boolean isUserExists()
    {


        SqlCommand cmduserName = new SqlCommand("select count(*) from userTable where userName= " + txtuserName.Text + ")", conn);
        SqlDataReader rdr = null;
        rdr = cmduserName.ExecuteReader();
       
        conn.Open();
      
        int count = 0;
        while (rdr.NextResult())
        {

            count = rdr.GetInt32(1);

        }

        conn.Close();
        if (count == 0)
        {
            return false;
        }
        else
        {
            return true;
        }
        
 
    protected void Button1_Click(object sender, EventArgs e)
    {

        if(isUserExists())
            {
                Response.Write("Opps ! User already Exists");
                        
              }
       
        SqlCommand adapInsert = new SqlCommand("insert into userName values('" + txtuserName.Text + "')",conn);
        conn.Open();
        adapInsert.ExecuteNonQuery();
        conn.Close();
        Response.Write("data inserted");

       }

 
 
is it write or not because I am not getting the output .
please tell me any one where I have to change the code
 
Thanks
Geeta