I'm sorry to hear you are having that problem! That seems like quite an odd issue.
Have you tried uninstalling Fiesta (in its entirety) rebooting, and then reinstalling it? That may resolve your issue. Outside of that, your anti-virus may be blacklisting the Fiesta Launcher executable and mistakenly labeling it as a virus or malware. Typically when this happens the executable is placed in quarantine or deleted from your computer as a means of protecting you. Try adding the launcher as an exception in your firewall / blacklist settings.
I hope that helps and good luck with playing!