If you installed "on top" without reformatting, then all you'd be replacing is the files that are installed by the installer - which means any changes you've made where you've created your own file would stay, but changes where you've changed an existing file would be lost. Many things like locatedb would be wrong, and... quite frankly, there are so many things that assume the system's set up in a particular way that it'd probably be easier to save your settings than try to correct the horrible old/new mismatch.
at a terminal, which will list all the mounted partitions on your system. The first one will probably be something like "/dev/hda1 on / type ext3", which is your root partition; if you have a like that says something like "/dev/hda2 on /home type ext3" (the "/home" is the important bit) then you do have a separate home partition, so it should be easy to reinstall, keeping your old settings. If not, you can just make a backup of your home directory, reinstall Ubuntu and then copy your backup back in.
FYI, if your problems came with installing KDE onto a Gnome-centric Ubuntu, you really should check out Kubuntu
. AFAIK it's exactly the same as Ubuntu, but with KDE instead of Gnome.