SharePoint won't know what to do if you simply create a host  header with a
new URL (it's tied to the address you used when setting up the
application). You need to set up an Alternate Access Mapping (AAM) via
Central Administration.

Open up CA in the browser and go to Operations. In the Global Configuration
section, you should see the "Alternate access mappings" link; click that and
select "Add Internal URLs". From there on, it should be fairly intuitive if
you're familiar with SP administration.

The main problem with the AAM is that i have to add it to the IIS and the IIS 7
is very difficult and i dont know from where to manage this?

Creating the host  header when you create the application  pool is fine. Make sure
you also add the DNS entry for this host or clients won't be able to find it.
You need to create AAM's if you are creating  additional host haeders to use with
the application pool. So, create the application pool  with whatever host haeder
you desire. Add appropriate DNS alias for the host header  and enjoy.

Can i make the host  header WWW.khaled.COM , cuz this is site  and i need the url
to be the same as the site .

missed the "when creating" part.

Pay no attention to the man behind the curtain.

Technically you can make it anything you want to, as long as you handle name
resolution. If you create the web application  with the url host  header of
www.khaled.com, then you create an alias for www that resolves to the host of
the system that is your server. You also need to be responsible for the
khaled.com domain.

I think this is more of a DNS understanding issue than a SharePoint issue.

I did this yesterday. If u dont have access to the DNS server then just edit ur
HOST file and add an entry.

[IP ADDRESS of MOSS server] [new name or alias]

Just be aware that this only works for the box where the HOSTs file is.
Changing the hosts file on the server won't help clients resolve the server.
For that you need DNS. It may solve the problem in a single development
environment where everything is on the one server.