After you install a custom Cryptographic Service Provider (CSP) on a computer that is running Windows XP, you cannot encrypt Office Open XML (OOXML) files or open encrypted OOXML files in the 2007 Microsoft Office system.
To resolve this issue, apply the following update:
2598030 Description of the 2007 Office system hotfix package (Mso-x-none.msp): February 28, 2012
Registry key information
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:
322756 How to back up and restore the registry in Windows
After you install the hotfix package, follow these steps to enable the hotfix:
Click Start, click Run, type regedit in the Open box, and then click OK.
Locate and then select the following registry subkey:
Microsoft Office Basic 2007, Microsoft Office Enterprise 2007, Microsoft Office Home and Student 2007, Microsoft Office Standard 2007, Microsoft Office Professional 2007, Microsoft Office Small Business 2007, Microsoft Office Ultimate 2007