When ever i tey to coonect ww.facebook.com it gives me error that u unable to cnnetc www.facebook.com..how ever i can open all other sies other then facebook... sooo plzzzzz help me out i also canged DNS .... i will b really thanful to u ....

Most probably the access to facebook is blocked from your windows or network firewall.
You or the administrator of the PC or network has to remove the RULE from firewall(s) that deny access to facebook site.

If above is not possible (e.g. in case it was made on purpose by the administrator, e.t.c.):
Alternative you can try to connect to facebook through proxies that can bypass your firewall.
Below you can see some of these proxies. Just go there, type www.facebook.com in the URL and press go. Whala!!!

http://www.facesaver.net
http://facoxy.com
http://vroxy.com
http://subick.com
http://midhat.com
http://vroxy.tk
http://vcloak.tk
http://ninjaoxy.tk
http://cloakme.tk
http://ktunnel1.tk
http://ktunnel2.tk
http://ktunnel3.tk
http://ktunnel4.tk
http://cloakninja.tk
http://proxyninja.tk
http://ninja-cloak.tk
http://ninjaproxy1.tk
http://www.airproxy.tk
http://ninjaproxyarea.tk
http://www.fastninjaproxy.tk
http://www.freeninjaproxy.tk
http://www.bypass-facebook.tk
http://www.getonfacebookatschool.tk