Compiler error in VisualStudio 2005

I am getting an error:
"The type or namespace name 'Tools' does not exist in the namespace 'Microsoft.Office' (are you missing an assembly reference?)      D:\TC4Rel23\Utilities\RestoreTestCaseKeywords.cs"

Error is pointing to the line "using Microsoft.Office.Tools.Excel;"

I looked at the "Reference" section of my Visual studio solution explorer and I saw Microsoft.Office.Tools.Common, Microsoft.Office.Tools.Excel and Microsoft.Office.Tools.Word are all showing with yellow exclamation sign..

I have installed .NEt programability for MS Office Word and Excel.
What could be the reason of the error? The code worked fine until last week.
Thanks.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;
using System.Collections;
using Microsoft.Office.Tools.Excel; //*******GETTING ERROR HERE******
using Microsoft.Office.Interop.Excel;
using System.Reflection;
using System.IO;
using System.Configuration;
 
 
Here is the error:
Error	1	The type or namespace name 'Tools' does not exist in the namespace 'Microsoft.Office' (are you missing an assembly reference?)	D:\TC4Rel23\Utilities\RestoreTestCaseKeywords.cs	6	24	TestApp

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