The "Invalid signature" indication appears when you try to open a Word document or an Excel workbook

When you open a Microsoft Office Word document or a Microsoft Office Excel workbook that has a digital signature, you cannot edit the Word document or the Excel workbook. Additionally, the "Invalid signature" indication appears even if the signature was created during the period when a certificate was valid.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
To open an Office file without displaying the "Invalid signature" indication, install the 2007 Office system hotfix package that is dated February 24, 2009, and then configure the IgnoreExpiredCert registry entry to enable the hotfix. To do this, follow these steps:

1. Install the 2007 Office system hotfix package that is dated February 24, 2009. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
   962217  (http://support.microsoft.com/kb/962217/ ) Description of the 2007 Office system hotfix package (mso.msp): February 24, 2009
2. Configure the IgnoreExpiredCert registry entry. To do this, follow these steps:
   1. Start Registry Editor.
      • In Windows Vista, click Start
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        , type regedit in the Start Search box, and then press ENTER.
        
        
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        If you are prompted for an administrator password or for confirmation, type the password, or provide confirmation.
      • In Windows XP, click Start, click Run, type regedit in the Open box, and then click OK.
   2. Locate and then select the following registry subkey:
      HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Common\Signatures
   3. On the Edit menu, point to New, and then click DWORD Value.
   4. Type IgnoreExpiredCert, and then press ENTER.
   5. Right-click IgnoreExpiredCert, and then click Modify.
   6. In the Value data box, type 1, and then click OK.
   7. On the File menu, click Exit to exit Registry Editor.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.