If a keyboard (regardless of manufacturer) is "Class Compliant", that means it will work with the default driver that has been built-in to Windows since XP Service Pack-2 and the last several versions of MAC-OS, and does not require the installation of an after-market or third party driver. The CTK-800 is NOT Class Compliant, and will require installation of a Casio proprietary after-market driver. This can be downloaded from:
Scroll down and click on "I agree", click on "Save File" and "OK" in the pop-up window, and give Windows a path to where you want it downloaded.
Unzip the downloaded file and run the resulting executable file (dpinst.exe) to install the driver.
The compliant list specifically shows the CTK-800 in the webpage header, but only shows Windows 7 and 8: 32 and 64 bit, but that is primarily because Windows 10 had not been released yet, when this webpage was designed. Some users have reported that they have been able to get this driver to work with Windows 10, while others report that they have not. The difference seems to be dependent upon the particular Windows 10 / hardware combination and is most likely a hardware BIOS and/or chipset issue, but it is certainly worth a try. Casio will assure you that it will not work with Vista 64 bit, but I have been using it just fine to connect my WK-3800 to my 64 bit Vista system since it was released several years ago. If it will install on your system, but does not work, it is just a simple matter of uninstalling it, but most of the time, if there is a problem, it will not even install to begin with.
Good luck - Hope it works for you !