Ubuntu 10.04 Lucid Lynx is uses the 2.6.x kernel and the server edition is supported until 2015-04. You can download it here - http://releases.ubuntu.com/10.04/
For more on the differences between server editions and desktop editions of Ubuntu, see this question on Ask Ubuntu.
The main issue seems to be that there is no desktop environment included in the default installation. As such there is no GUI installation, although what they give should be intuitive enough to use. You will get other packages installed which you usually get on a server too. Lucid is also old enough to have a server optimised kernel, I'm not sure what the exact differences are but they should me minor enough not to noticeably affect anything.
It should also be ok to install the desktop edition too, it can be downloaded here - http://old-releases.ubuntu.com/releases/10.04.3/ (get a 10.04.4 download for more included updates). The repositories are the same for both anyway, it is just that 'server support' probably means that only the server relevant packages are updated. For example the server optimised kernel will probably get security updates while the desktop kernel won't.