Hi Nada,
I tried to make this clear in my privous post. You said you understood but i think you did not.
Yes, other people in your building are using channels 1, 6 and 11 already. However you should also use these channels. If you use channels 5 for example, this channel over laps with both channels 1 and 6. You will get interference from other access points on both of these channels, and it will cause your channel 5 to have even more issues then if you were just on channel 6. You are not helping yourself at all by using these channels.
Again, see here: http://upload.wikimedia.org/wikipedia/commons/thumb/8/8c/2.4_GHz_Wi-Fi_channels_%28802.11b,g_WLAN%29.svg/2000px-2.4_GHz_Wi-Fi_channels_%28802.11b,g_WLAN%29.svg.png
If you use channel 2-5 all of those channels will hear interferenc form channels 1 and 6. If you use channels 7-10 they will receive interference from channels 6 and 11.
By using 1, 6, and 11 your APs have a full 20 mhz channel without overlaping themselves. Settings it to channel 5 does not help you with avoiding interference from neighbours on channels 1 or 6, it will make it worse as now you can hear two APs causing even higher noise.
I've designed WLAN networks for large corporate towers and even in those cases with many interfering neighbour APs you should only ever use the non-overlapping channels.