Hi Carlos,
This is very strange. I have just checked this and the installation package for build 341 does not have the zero key problem. I have uninstalled and reinstalled it and all works fine.
I suggest you to reboot your computer after uninstalling PixInsight. Then install 341, and there should be no problem at all.
Are you running Vista or XP? On Vista, better log in as administrator to uninstall and install PI.
Hope this helps. Please let me know.