Cant believe you've just posted this Lau - we've just been raking out some rotten wood on a post upright that we needed to fill, and to our amazement it was home to a nest of tiny ants.
We've had to turf them out. Thankfully though, we havent had an ant problem in the house. I've heard that the traditional method for killing off ant nests safely was to pour boiling water into the exit holes.
You can also buy sticky strips which you can put on threshholds and carpet edges, but I'd be less happy using those where children and pets play, just to be on the safe side.