Squeaky Floors on 2nd Floor

We live in a 3-bed two storey semi-detached house. The floor in one of the rooms upstairs squeaks audibly when somebody walks normal on it. Just wondering if anyone out there knows what causes this squeaking or how one might fix this problem?

Its kinda annoying when you are watching TV and someone overhead is walking around.

thanks Gallogly

It's probably a loose or warped floorboard. If you can identify it, then a few nails will probably to the trick.