We have a form that requries full trust that has been signed with a certificate and published to a SharePoint Form library. Form has worked beautifully for all my users until today. Today, a user tried to open an exisitng instance of the form and recieved the message "The signature is corrupt or the certificate used to sign this form is expired". He also gets the message when trying to create a new instance of the form. The rest of my users continue to interact with the form as expected. This was the first time he's ever tried to interact with this form (or any signed form). I've seen the article http://support.microsoft.com/kb/888704 and I'd be willing to buy it if all my users were experiencing the issue, but so far, its just the one. Any thoughts?