[Ignignokt]

First, you MUST find out where they're coming from. If necessary, put out some honey and observe the trail of ants.

Then, get some ant bait. Very cheap (~$5-7 at Home Despot). Put it in the direct path of the ants, preferably immediately where they enter the house. (If you have a cat/dog, make sure it isn't messing with the bait. Sniffing shouldn't harm it, but chewing on them may.)

I've had ants twice in three years, and both times the bait worked within 24 hours. They take it back to the nest and all the ants eat it, including the queen, and they die.

Any solution is only temporary - ants will often have many colonies in your area, and eventually they'll return seeking food and/or a place to live. But this one has worked very well for me.