Thank you for contacting Funcom Technical Support.
At this time, we do not support Windows 8.1. Some of our customers have been able to successfully play The Secret World on 8.1, however there have been others who have encountered severe compatibility and performance issues.
For this reason, we do recommend waiting on the upgrade until we have full code support for Windows 8 and Windows 8.1.
Thank you for your interest, and please have a great day!