Due to the increased security in Windows 7 and Vista if you require a portion of the HTTP URL namespace (i.e. http://+:80/MyPortion) Then It must be assign by an administrator from the command line. This is of particular importance to those implementing Jpip or Wado servers.
The Command to assign the http portion is
netsh http add urlacl url=http://+:80/MyUri user=DOMAIN\user
For more details please see the MSDN page