Would like to thank you for some excellent replies. @AR., your a star and it works perfectly. I had noticed last night that the Excel.exe was not closing; so I did some research and found out about how to release the COM objects. Here is my final code:
using System;
using System.Collections.Generic;
using System.Text;
using System.Reflection;
using System.IO;
using Excel;
namespace testExcelconsoleApp
{
    class Program
    {
        private String fileLoc = @"C:\temp\test.xls";
        static void Main(string[] args)
        {
            Program p = new Program();
            p.createExcel();
        }
        private void createExcel()
        {
            Excel.Application excelApp = null;
            Excel.Workbook workbook = null;
            Excel.Sheets sheets = null;
            Excel.Worksheet newSheet = null;
            try
            {
                FileInfo file = new FileInfo(fileLoc);
                if (file.Exists)
                {
                    excelApp = new Excel.Application();
                    workbook = excelApp.Workbooks.Open(fileLoc, 0, false, 5, "", "",
                                                        false, XlPlatform.xlWindows, "",
                                                        true, false, 0, true, false, false);
                    sheets = workbook.Sheets;
                    //check columns exist
                    foreach (Excel.Worksheet sheet in sheets)
                    {
                        Console.WriteLine(sheet.Name);
                        sheet.Select(Type.Missing);
                        System.Runtime.InteropServices.Marshal.ReleaseComObject(sheet);
                    }
                    newSheet = (Worksheet)sheets.Add(sheets[1], Type.Missing, Type.Missing, Type.Missing);
                    newSheet.Name = "My New Sheet";
                    newSheet.Cells[1, 1] = "BOO!";
                    workbook.Save();
                    workbook.Close(null, null, null);
                    excelApp.Quit();
                }
            }
            finally
            {
                System.Runtime.InteropServices.Marshal.ReleaseComObject(newSheet);
                System.Runtime.InteropServices.Marshal.ReleaseComObject(sheets);
                System.Runtime.InteropServices.Marshal.ReleaseComObject(workbook);
                System.Runtime.InteropServices.Marshal.ReleaseComObject(excelApp);
                newSheet = null;
                sheets = null;
                workbook = null;
                excelApp = null;
                GC.Collect();
            }
        }
    }
}
Thank you for all your help.