First to Don, if you have sagging floors in a one year old dwelling, you might have a serious structural problem. No floor in a two story dwelling should have a recognizable 1 1/2" deviation in any room, it is completely unacceptable and points to structural failure. Immediately contact a local building official to address the issue.
Second to Steve. Your floors were constructed without glue and with nails and not screws. If you can get to the bottom side of the floor, you can apply liquid nails to the edges of the floor joists (each side) to reduce the problem. Use plenty of glue and start on only the squeaky areas. There are other fixes, but none are simple.