Try the following solution please:
Right-click on the SWF & FLV Toolbox icon on your desktop and select "run as administrator". NOTE, it is not the same as being administrator at this computer! Now register SWF & FLV Toolbox with your Registration info as you did before. After this action you can run SWF & FLV Toolbox as before and the issue should be gone.
If the problem still persists, please view My Registration key doesn’t work article.