***Problems in ubuntu local network installation

Hi,
I am an ubuntu user.I tried to install ubuntu in 4 computers(same architecture) with a minimum effort.So,I did network installation.It was going fine in ubuntu 5.1 distro.But when i tried with ubuntu 7* distro it is not getting over.When i am selecting my local repo. path (mounted iso image in the local server),it is giving the following error... "The specified ubuntu archive mirror is either not available,or doesnot have a valid Release file on it.Please try a different mirror." But at the same time when i am selecting official ubuntu mirror it is going fine..
Once again i am mentioning that i am using the same procedure that i followed during ubuntu 5.1 distro network installation.and it worked fine. My intention is to install ubuntu from (mounted install cd) local server without using the internet.
I followed the PXE local network installation procedure.In my server i am using dhcp3 server,tftpd-hpa for tftp,and apache2 for http. Another thing i want to ask is that is it a problem of ubuntu-installer?As i found that in ubuntu 5.1 distro netboot option is there in the install cd itself (/install/netboot) and it is mandatory for PXE net installation but in ubuntu higher versions they are not providing this option in the install cd (or in the iso image) so i downloaded the netboot.tar.gz package from the official ubuntu repo and untar it inside my tftp root directory(/var/lib/tftpboot).So that my computers are now booting from the server but the ubuntu-installer fails in that stage what i mentioned above. I searched ubuntu docs and forums but yet to get any suitable answer. Please help me at your earliest.Any comments on this regard would be appreciated.
Thanks & regards,
S.Pahari.

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