What causes squeaky floors?
Floors squeak because something is moving. The main culprits are:
the edges of sheets of subfloor rubbing against each other (because loose nails are allowing the boards to move)rubbing between tongue and groove wood that has dried a little over time leaving a gap for movement floor joists shrinking away from floorboards overtime leaving a small gap that squeaks when you put weight on it.
The fix is simple, stop the movement and you will stop the squeak. You stop the movement by nailing or screwing the boards, shimming under the joists or adding bracing between the joists. The challenge is to figure the cause and get access to fix it.
Read more:
http://www.doityourself.com/stry/silencesqueakyfloor#ixzz1oLzG5jHJ
Source: http://hmdhome.com/whats-included.php