'D:LifeCycleDemo/Uploads/Data Mapping & Assessment Report.doc' is not a valid virtual path.

string filePath = (sender as LinkButton).CommandArgument;
Response.AppendHeader ("Content-Type", "Application/octet-stream");
Response.AppendHeader("Content-Disposition", "attachment; filename=" + Path.GetFileName(filePath)  );
Response.WriteFile(filePath);
Response.End();