We test each SpiderOak ONE release on Windows 7 (32 and 64 bits), 8.1 (64 bits), and 10 (64 bits). These are the Windows versions for which we provide official support.
We regularly receive reports of ONE running fine on Windows Server and SBS versions 2008 and later, Vista, 8, and on the 32 bit versions of 8.1 and 10. We do not however test on them and so cannot offer advanced troubleshooting for them.
Windows XP is not supported, and we echo Microsoft's encouragement that XP users upgrade to a more recent and secure operating system. In the meantime, XP users have reported that ONE version 6.1.5 continues to work for them. That is an older and thus unsupported version of ONE, but if you would like to try it on an as-is basis then here are the installers:
If a more recent version of ONE was installed by accident on a computer running Windows XP, ONE will not work until downgraded to 6.1.5. To do that, open the Install/Uninstall Software applet of the Windows Control Panel and uninstall SpiderOak ONE. Reboot the computer. Download the appropriate version of ONE 6.1.5 from the links above and install it.