Distiller problem in VB.NET

'Author:    Tim Wuytens
'Goal:      Convert *.ps to *.pdf using adobe acrobat distiller automation
'Date:      2005-10-25
Imports System.IO
Imports ACRODISTXLib
Public Class frmDistillerAPI_Converter
    Inherits System.Windows.Forms.Form

#Region " Windows Form Designer generated code "
'omitted code for form generation
'it's a simple form with a textbox, a listbox and 2 buttons(start and browse)
#End Region
    
'Declare object with its events
Public WithEvents objDistil As PdfDistiller6

'Handler for the Click event on the Browse button
'Shows a filedialog where you can select one or multiple files
'and puts these filenames in a listbox
Private Sub btnBrowse_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) _
Handles btnBrowse.Click
    Dim str As String
    With OpenFileDialog1
        .CheckFileExists = True
        .CheckPathExists = True
        .Multiselect = True
        If DialogResult.OK = .ShowDialog Then
            For Each str In .FileNames
                lstPad.Items.Add(str)
            Next
        End If
    End With
End Sub

'Handler for the Click event on the Start button
Private Sub btnStart_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) _
Handles btnStart.Click
    Dim InputPS As String
    Dim str, OutputPDF, JobOptions As String
 
    'set reference to new object       
    objDistil = New PdfDistiller6

    'iterate through all listitems
    For Each str In lstPad.Items
        InputPS = str
        OutputPDF = InputPS.Substring(0, InputPS.LastIndexOf(Path.DirectorySeparatorChar))
        OutputPDF &= "\pdf\"
        If Not Directory.Exists(OutputPDF) Then
            Directory.CreateDirectory(OutputPDF)
        End If

        'catch result and show in textbox
        Dim res As Short
        'the FileToPDF function does the actual conversion
        'it works fine, even when multiple files are selected
        'however, when the button is clicked again
        'the exception mentioned above is thrown
        'I know for sure this is because the acrodist.exe process is still running
        'because I tried manually ending it between two clicks, and then
        'it works fine again
        res = objDistil.FileToPDF(InputPS, OutputPDF, JobOptions)
        txtUit.AppendText(res)            
    Next
        
    'release the object reference
    '(was meant for ending the process acrodist.exe, but doesn't do its job)
    objDistil = Nothing
End Sub

'OnLogMessage catches the logmessages that normally would appeat in the distiller log
Private Sub objDistil_OnLogMessage(ByVal strMessage As String) Handles objDistil.OnLogMessage
    strMessage = strMessage.Replace(vbLf, vbNewLine)
    txtUit.AppendText(strMessage)
End Sub

End Class