I had not been able to access the RCI portal through HGVC using IE with Windows XP SP2 and Vista. I sent a message to HGVC and received this response today:
"Thank you for contacting Hilton Grand Vacations Club. First and foremost, please accept our sincere apology for the error you encountered when attempting to load the RCI booking system. It is still being updated with new information. You will soon have full access to RCI booking engine, in the meantime you can contact a Club counselor directly at 800-932-4482 or +1 (407)722-3141 to check availability or book reservations. We apologize for any inconvenience this may cause you.
Please let us know if you have additional questions pertaining to your account and we will be happy to assist you. "
In the meantime...yesterday I saw on the Wyndham location that someone suggested clicking on the "Compatability View" (looks like a torn paper) at the top of the page next to the address bar. I tried this and I have been able to access RCI ever since.