Ok.
I built de light on another computer which is more capable. It worked.
From default C:\Users\mail_000\AppData\Local\UnrealEngine\4.2\Saved\Swarm i copied SwarmCache Folder to my computer in default C:\Users\mail_001\AppData\Local\UnrealEngine\4.2\Saved\Swarm
Now lightbuilding seems to be working fine.
Thank you for the support Eric Ketchum.