Vista - Word Automation on Custom Properties

Hi,
I´m trying to make word automation on document custom properties(for a given
property name change the value)

I put this code together:
Type Prop = Type.GetTypeFromProgID("Word.CustopParoperties");
System.Object obj = Activator.CreateInstance(Prop);

Microsoft.Office.Interop.Word.CustomProperties CustomProp = obj
as Microsoft.Office.Interop.Word.CustomProperties;
foreach (Microsoft.Office.Interop.Word.CustomProperty property
in CustomProp)
{ if (property.Name == valfind) property.Value = valreplace; }

and I get this error

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: type

Source Error:


Line 53:
Line 54: Type Prop =
Type.GetTypeFromProgID("Word.CustopParoperties");
Line 55: System.Object obj = Activator.CreateInstance(Prop);
Line 56:
Line 57: Microsoft.Office.Interop.Word.CustomProperties
CustomProp = obj as Microsoft.Office.Interop.Word.CustomProperties;


As I understand I dont have the ProgID right
any ideas what should be there
I´m quite new to this and running out of hope

Thanks