Enumerator Performance Issue (Needs a better method)

Based on the first impression of your code: Accessing properties of an UI control is always slow. Since you're iterating through files , in my opinion you don't have to set the texts etc to the controls, use variables instead. This also applies to textbox1 and ...2. Since textbox 2 is 'static in during your iteration, take the value from the Text property to a string variable and use that in your loop.

Also since you fill a list box in your code, don't add the items as you go but gather them into a collection and after the loop, use ListBox.ObjectCollection.AddRange Method[^]

Addition:

Most likely the code I wrote is wrong since I didn't know the whole idea for the code, but perhaps something like the following:

...
        Dim text1 As String
        Dim text2 As String
        Dim diTop As New System.IO.DirectoryInfo(ComboBox1.SelectedItem)
        Try
            ' Enumerate all subdirectories.
            For Each di In diTop.EnumerateDirectories("*")
                Try
                    ' Enumerate each file in each subdirectory.
                    For Each fi In di.EnumerateFiles("*", System.IO.SearchOption.AllDirectories)
                        text1 = getMd5Hash(fi.FullName)
                        If text2.Contains(text1) Then
                            itemList.Add(fi.FullName)
                        End If
                        Try
                            ' Catch unauthorized access to a file.
                        Catch UnAuthFile As UnauthorizedAccessException
                            Console.WriteLine("UnAuthFile: {0}",
                                                UnAuthFile.Message)
                        End Try
                    Next
                    ListBox2.Items.AddRange(itemList.ToArray())
                    ' Catch unauthorized access to a subdirectory.
                Catch UnAuthSubDir As UnauthorizedAccessException
...