Error when open word application in ASP.NET

System.Runtime.InteropServices.COMException: Cannot activate application

I have the namespace Microsoft.Office.Interop.Word included in my program

The Code is like this

Microsoft.Office.Interop.Word.Application word = null;
Microsoft.Office.Interop.Word.Document doc = null;
word = new Microsoft.Office.Interop.Word.Application();
object falseValue = false;
object trueValue = true;
object missing = Type.Missing;
word.Visible = true;
word.Activate();
doc = word.Documents.Open(ref fileName, ref missing, ref trueValue, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);
doc.Activate();

I get error on the line word.Activate();

Please help