Код:

Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        Dim reader As New StreamReader("text.txt", System.Text.Encoding.GetEncoding(1251))

        Do While reader.Peek() >= 0
            Dim strok As String = reader.ReadLine()

            For Each Text As String In strok
                If Text.Equals(TextBox1.Text) Then
                    MsgBox("Такое слово уже есть")
                End If
            Next
        Loop
        reader.Close()

    End Sub

End Class

Код:

For Each Text As String In strok
    If Text.Equals(TextBox1.Text) Then
        MsgBox("Такое слово уже есть")
    End If
Next