carpet on basement floor
First off, I do not reccomend carpet in a basement. The relative humidity is generally too high in most basements and the carpet will end up being musty.
If you are determined to put carpet in the basement, you must first determine if there is drive (water vapor) coming through the slab and how much is coming through.
An environmental engineer in your area can perform a dome test for you to calculate the amount of drive.
An easy test to check for presence of drive is too tape 2' square of plastic to the floor. tape all of the edges so that the plastic is sealed. Check in a couple of days to see if water has accumulated on the underside of the plastic. If is has go to the dome test to see if the drive is inside of the flooring manufacturers guidlines.
Hope this helps,