machine name : win2003
domain name : hello.com
I created new alias (CNAME) :
alias name : my
fully qualified domain name for target host : win2003.hello.com.

in Central Administration site after I did New Web Application ,Site Collection Administrators ,Web Application General Settings ,Create Site Collection....etc ,then when I wanted to enter the path to the master Site Directory where I would like to capture
I type the site: http://my.hello.com
but it gives me an error message "This not a valid site directory. Specify a URL to a site that is using the site directory template."
I don't know how to fixed or where is the error and what did I make mistake?

so..I need your help