This problem is fixed in the 2007 Office system hotfix package that was released on November 19, 2008.For more information about the hotfix package, click the following article number to view the article in the Microsoft Knowledge Base:
Description of the 2007 Office system hotfix package (Mso.msp): November 19, 2008
After you install this hotfix package, the Choose a digital certificate
dialog box appears in front of all windows as expected. Additionally, this hotfix package does not affect any other dialog box prompt that might occur as the result of other configurations.