Vista users unable to check out at our site

Hi all. We have a problem with our ECommerce web site and Vista users. When they get to Checkout an error pops up sayiing that the certificate is in error. The user can get the certificate exception if they know how but most users will now know how to do this nor can we expect them to. My question is this: Is there a work around for this on the certificate level? It would be unreasonable to expect us to try and teach our customers to try and modify Vista prior to going to Checkout so it must be on the certificate level if a workaround is to be had.