Our house is about 30 years old and when we are downstairs and someone is walking up stairs you can hear the floors making this squeaky noise. What is causing this noise? We are planning on replacing the carpet but wondering if we need the floors repaired? If we need them repaired, what kind of repairs? About how much would it cost?

If these are fairly minor sqeaks its most likely just sub floor that's worked loose a little over time and what you are hearing is the sub floor moving. If your having your carpet replaced soon you can have your carpet installer take a look at it. Most of them around here are more than happy to put some extra screws into floor joists to secure loose areas of sub floor .Once the old carpet and pad come up at least you can either inspect or have the carpet installer inspect the condition of your floor before new installation is done.

AC101 describes the situation quite well. It could be the sub floor but do you know if there is finished hardwood under the carpet? The tongue and groove hardwood can make a lot of noise if it has shunk over time. In any case, once the old pad is up all repairs can be done. If it's a sub floor you can just screw in 2.5" screws with a square head or combo head. If its hardwood you can use the same screw but should drill a pilot hole the size of the screw shaft and countersink the head.

I'm having the same problem in the master bedroom on the main floor. Our house is 35 years old, and I remember the previous owner who sold the house to us 25 years ago, mentioned something about having two plywood floors on the joists. He said he didn't like the quality of the 1/2" plywood the contractor installed due to knots etc, so the contractor added another sheet of 1/2" plywood throughout the whole main floor.
I removed the carpet in our bedroom cause I'll be installing a 12mm thick laminate floor x 3 5/8" wide boards. I had made myself a promise years ago that the next time we replace the floor I was going to re-screw the whole room again. (didn't do it the last time we replaced the carpet)
Anyway, I've screwed and nailed right into the joists, which are 12" centered, and there is one spot between two joists that is really squeaking and I just can't get rid of it. I went down to the basement and removed the suspended ceiling tiles, to check if the joists were bowed etc, and to make sure I was screwing into the joists, but I don't see anything that looks wrong. To make sure, I tried adding a few cedar shims between the floor and the joists, but I couldn't, as things were good an tight. I even took a flashlight to see if light would go through at the top of the joists and I didn't see light on the other side. The last thing I just tried was, screwing some 2 x along side the top of the joists butted against the top floor, but that didn't help, so I screwed up from underneath the plywoods and through the joists hoping it would pull the plywood down, but that didn't work either. I am really stumped by this, and for the life of me, I can not get rid of the bad squeak in that one area, [which is an area I usually walk on to get to and from my bed].
Is there anything else I can do to solve this problem or is the problem caused by the two plywoods, and I'll have to live with it ?
Very annoying when you can feel the noise on the joist, yet can't tighten things up. Maybe there's some form of bracket they sell to fix this type of thing.