Excel could not be found - missing assembly reference?

Hi, I did get this error in my error list while compiling the project. Previously the project are done with MS Office Excel 2003, and currenlty i am using Excel 2007. In my system i don't have MS office 2003. In my project, my references i did add Excel and Microsoft.Office.Core.

The error is ->
The type or namespace name 'Excel' could not be found (are you missing a using directive or an assembly reference?)      

private int findColumnDept(Excel.Worksheet Ws)

Error was underline in "Excel"

private int findColumnDept(Excel.Worksheet Ws)
		{
		
			int no=1;
			string CellValue="";
			int retryCount=0;
			bool EmptyCellAllowed=true;
			do 
			{
				//listBox1.Items.Add(worksheet.getCells[i,1].ToString());
					
				Excel.Range range1 = Ws.get_Range( "A"+no, "A" + no);
				no++;	
					
					
				
				if (range1.Cells.Value2!=null)
				{
					CellValue=range1.Cells.Value2.ToString().Trim();
					EmptyCellAllowed=false;

				}
				else
				{
					retryCount++;
					CellValue="";
					if (retryCount>15)
					{
						EmptyCellAllowed=false;
						no=0;		
					}
					
				
				}
			}while ((CellValue.Length>0)||(EmptyCellAllowed)) ;
			

			return no-1;

		}

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