Who can fix problem with the download server - wrong package in package lists

Hello,
whenever I do "apt-get install build-essential" I got the message : "Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/l/linux-source-2.6.20/linux-headers-2.6.20-16-server_2.6.20-16.28_i386.deb 404 Not Found"!

I'm running Ubuntu 7.04 - kernel 2.6.20-16-server. On the Ubuntu-Package-Server it gives a meta-package called "linux-headers-server(2.6.20.16.28.1) [security]" but that shows to "linux-headers-2.6.20-16-server" this includes "linux-headers-2.6.20-16-server(2.6.20-16.29) [security]". You see I need the version 2.6.20-16.28 and can only get version 2.6.20-16.29.

Code:
 apt-get install linux-headers-2.6.20-16-server
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  gcc-4.1 cpp gcc dpkg-dev cpp-4.1 patch binutils make
Use 'apt-get autoremove' to remove them.
The following extra packages will be installed:
  linux-headers-2.6.20-16
The following NEW packages will be installed:
  linux-headers-2.6.20-16 linux-headers-2.6.20-16-server
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 8966kB of archives.
After unpacking 65.5MB of additional disk space will be used.
Do you want to continue [Y/n]?
Err http://security.ubuntu.com feisty-security/main linux-headers-2.6.20-16 2.6.20-16.28
  404 Not Found
Err http://security.ubuntu.com feisty-security/main linux-headers-2.6.20-16-server 2.6.20-16.28
  404 Not Found
Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/l/linux-source-2.6.20/linux-headers-2.6.20-16_2.6.20-16.28_i386.deb 404 Not Found
Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/l/linux-source-2.6.20/linux-headers-2.6.20-16-server_2.6.20-16.28_i386.deb 404 Not Found
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

What can I do? Please help.

regards Axel