Public Shared Function acrobat_vorhanden() As Boolean
        Const erweiterung As String = ".pdf"
        Dim reg As RegistryKey = Registry.ClassesRoot.OpenSubKey(erweiterung)
        If reg Is Nothing Then
            Return False
        Else
            Return True
        End If
    End Function

 Dim regKey As RegistryKey = Registry.CurrentUser.OpenSubKey( _
   "Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts")

Dim programs() As String
Dim regKey As RegistryKey
regKey = Registry.CurrentUser.OpenSubKey("Software\Microsoft\Windows\" & _
"CurrentVersion\Explorer\FileExts\.pdf\OpenWithProgIds")
 
If Not regKey Is Nothing Then
   programs = regKey.GetValueNames
     For i As Integer = 0 To programs.Length - 1
         MsgBox(programs(i))
     Next
End If