Ok, I found your solution, your drive string contains 'C:' you need to append the '\' to it.
Dim drive As String = Path.GetPathRoot(System.Environment.ExpandEnvironmentVariables("%SystemDrive%")) & "\"
Dim counter3 As System.Collections.ObjectModel.ReadOnlyCollection(Of String)
counter3 = My.Computer.FileSystem.GetFiles(drive, FileIO.SearchOption.SearchAllSubDirectories, "*.tmp")
Label1.Text = (CStr(counter3.Count))