Hi,
Make sure that your customers have the Framework 1.1 installed. Then, make sure that Office (or InfoPath alone) is installed on top of the Framework. If the Framework is installed AFTER Office, you need to install the Office 2003 PIA:
http://www.microsoft.com/downloads/details.aspx?familyid=3c9a983a-ac14-4125-8ba0-d36d67e0f4ad&displaylang=en
Hope that it helps.
Regards,
Franck Dauché