Java SE (Java Platform, Standard Edition)

I have a Java applet (called PALS) that was developed with version 1.5 update 7. It has been signed with a digital certificate from Thawte. Recently a customer had her Windows XP machine re-imaged, and that image comes with the Java 6 JRE. Others have had this done and were still able to run my applet, but she gets an error message that complains about an "invalid keystore format."

I tried going into the Java Console and removing the user's PALS certificate, but it also complains about the keystore format. I then tried just removing Java 6 and installing Java 5, hoping that that would would clear things up, but she gets the same error.

Am I correct that when a user runs my applet the certificate is read from the JAR file and placed in a keystore on their machine? If the keystore has the wrong format, can I just delete it, and where is that file? I thought it was supposed to have the name .keystore but I can't find that anywhere.