just got a windows 8 computer and carnt play ffxi PLEAS HELP

awesome guys this is great help thanks, ill try this out as soon as play online has stopped updating.
i messed up and was playing around with the properties on the FFXI icon on steam and clicked verify integrity of game cache and now i have had to reinstall FFXI from the beginning because every time i tried to start FFXI after this there was an error saying product key not found and another error saying FFXI was not installed correctly.
Thanks a lot guys as soon as i know if using windowed mode has worked ill post on here thanks again :-)