They winter in a protected place then swarm looking for a place to set up their summer home. They might be nesting in your home, or in a rotten log or partially buried piece of wood. Here is a good link with good info about them.
http://www.extension.umn.edu/distribution/housingandclothing/DK1015.html Carpenter Ants During Spring
It is common to find carpenter ants in homes during spring. It is important to try to determine whether the ants are coming from an outdoor or an indoor nest, although this can be difficult. Their presence is not sufficient evidence to conclude that there is a nest in your home. You may be able to make a more accurate determination based on when you first see carpenter ants. If you find carpenter ants in your home during late winter or early spring, that suggests the ants are coming from a nest in the building. However, if you see activity later in the year, it is less clear if the nest is in the building.
You may also see carpenter ant swarms (i.e. the reproductive queens and males, figures 1 and 4) during spring. Carpenter ants produce large numbers of queens and males during late summer. They emerge from nests the following spring for their nuptial flights. After mating, queens search for suitable sites to begin new nests. Once they land, their wings break off and each queen attempts to construct a new nest.
When carpenter ant nests are indoors, mating swarms become trapped inside. Finding large numbers of winged ants indoors is a sure sign that an indoor nest exists and may give the approximate location of the colony.
Finding one to several winged queens (figure 4) does not automatically mean a nest is present indoors. It is more likely the queens have just mated and have entered the home, searching for nesting sites. Wingless queens (figure 1) found walking indoors are new queens that have recently shed their wings but are still searching for nesting sites. They are not an indication of an indoor nest. (much more @ link)
They are very interesting creatures.