Visual Basic

Private Sub BGWorker_DoWork(ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BGWorker.DoWork
                
                For Each file As String In My.Computer.FileSystem.GetFiles("c:\", FileIO.SearchOption.SearchAllSubDirectories)

                    'Check for cancelation of Background worker.
                    If BGWorker.CancellationPending Then
                        e.Cancel = True
                        Exit Sub
                    End If

                    Dim extn As String = IO.Path.GetExtension(file)

                    Try
                        Dim sql As String = "Select * from ThreatType"
                        Dim cmd As New OleDb.OleDbCommand(sql, con)
                        ThreatDatabase.Connect()

                        Dim reader As OleDb.OleDbDataReader = cmd.ExecuteReader

                        While reader.Read
                            If extn.ToUpper = reader.GetString(0) Then
                                If IO.Path.IsPathRooted(file) Then
                                Me.RefreshList(file, reader.GetString(1)) 'cross-threading
                            End If
                        End While
                        reader.Close()

                    Catch ex As Exception
                        MsgBox(ex.Message)
                    End Try
                Next
            End If
        Next
    End Sub

Delegate Sub UpdatetList(ByVal [Name] As String, ByVal [Type] As String)

Private Sub RefreshList(ByVal [Name] As String, ByVal [Type] As String)
        If frmMain.ListView1.InvokeRequired Then
            Dim d As New UpdateList(AddressOf RefreshList)
            frmMain.ListView1.Invoke(d, New Object() {[Name], [Type]})
        Else
          frmMain.ListView1.Items.Add([Name]).SubItems.Add([Type])
        End If
    End Sub

Me.ListView1 

frmMain.ListView1 

Public Class Form1
    Dim FILE_NAME As String = ("serial.txt")

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

        Dim objReader As New System.IO.StreamReader(FILE_NAME)

        Do Until objReader.EndOfStream
            If txtserial.Text = objReader.ReadLine Then
                Startnot.Checked = False

                Form2.Show()
                Me.Hide()
            Else
                Try
                    If txtserial.Text = objReader.ReadLine.Trim Then

                    End If

                Catch Ex As Exception


                End Try
                Label1.Text = "Invalid CDKey"
            End If

        Loop
        objReader.Close()


    End Sub

    Private Sub Form1_Leave(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Leave
        If Me.Visible = False Then
            Form2.Show()
        End If
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        SetAttr(FILE_NAME, FileAttribute.Normal)
        Dim Dontshow As Boolean

        Dontshow = GetSetting(My.Application.Info.ProductName, "Options", "Dont Show Me", 1)

        If Dontshow = 0 Then
            Form2.Show()
            Call cookies()
            Exit Sub
        End If

        Startnot.Checked = True
    End Sub

    Private Sub Startnot_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Startnot.CheckedChanged
        SaveSetting(My.Application.Info.ProductName, "Options", "Dont Show Me", Startnot.CheckState)
    End Sub

    Private Sub cookies()

        Me.Close()
        My.Forms.Form2.Show()
    End Sub

End Class

I'd start by scrapping what you have and replacing it with a seperate class. You main form should call this class, call it "Licensing", to ask if it is OK to run the app. The Licensing class has to do a few things to answer this question.

First, it has to check to see if the app has already been through the CDKey entry process and that a valid Key was entered. This could be a value written to the registry or to your AppSettings file.

If the value doesn't exist or it indicates that the license is not valid, this class would put up a new form, or even just an InputBox, asking for the CD key. Once the key is entered, it is compared against the first line of the serial.txt file. If they match, then you create/update the value tracking this in the Registry or your AppSettings file. If not, then Licensing doesn't change anything and returns False to say that it is not OK to run the app.

If the value is OK, then the method in your Licensing class would just return True to say that it is OK to run the app. There's no need to put up the CD Key entry form again.

You can get the first line of the file using:
    Dim keyInFile As String    ...    Using sr As New StreamReader(Path.Combine(Application.StartupPath), "serial.txt"))        keyInFile = fs.ReadLine()    End Using