I'm no guru but have learned a bit over my year of adventures with Ubuntu.
Your system specs probably don't matter in this case.

Have you tried logging in at the prompt provided by what's left of your old
system? If it says login just type your user name, press enter and when
prompted type your password.

If you get that far it would probably be easiest to go back to your KDE
desktop (at least for now). If you have internet acces you should be able to
do so by typing:

sudo apt-get install kubuntu-desktop

(please someone correct me if the name of the desktop is different)

If should be possible to use a similar line to re-install the Kubuntu
desktop from the original CD (not the Mint one). That should replace enough
to get you into reasonable shape.

Ok...the first thing I would do is go to run level 1, which is single
user mode, since Fedora/Redhat has an init - run level 3, which gives
you a network connection, but no X-server you can probably get there
through the telinit command, or just plain init in Debian/Ubuntu
version of this bold new adventure in Operating Systems, an OS I have
been using since the 1990's (Red Hat 6.0).

So when you boot go into maintance mode, init 2, if you just get the
login prompt with no X-server, and as was mentioned by another poor
soul who also has yet to admit that the best software costs money, do
an apt-get install kubuntu. Personally I prefer not to use Kubuntu,
as Gnome is more stable, Sun actually paid Nathan Friedman and Miguel
Del IgCaza (sp??) to develop this desktop manager and UI.

Windows XP and 7 really is a nice OS, and I can still play all my
cool games. I love Ubisoft stuff, and there is something to be said
for people paid to write software.

AMD 4800+ X2 processor
4 GB 667Mhz RAM
Nvidia 8500 GT graphics card, 1024 MB Memory
Soundblaster X-Fi Extreme Music -> digital I/O module->spdif to my
Denon receiver and real speakers (Klipsch Cornwalls and Klipsch
Heresy III's), not computer toys.
* Guy who dual boot Kubuntu studio or Windows XP (I like to game)

When removing a desktop, such as Kubuntu, you should first install Gnome and
logoff. enter Gnome from the sessions at the bottom of the logon screen.
complete you de-installation of Kubuntu from there. It is the same for any
desktop. Make sure that you are out of the desktop that you are attempting to
remove and that your new one is working first. You can have as many desktops
installed as you want and can switch between them before you login.

The most likely source of your problem is that you removed KDM before GDM was
setup properly. Removing KDM used by Kubuntu would remove your desktop manager,
but it would stay in memory because it is in use. GDM would be installed with
Ubuntu, but you would need to explicitly tell it to use GDM as GNOME would use
KDM which was previously installed and in use in memory. When you logout, KDM
would be shutdown and lost from memory. GDM would not start up because Gnome
would not be configured to use it. If you type startx it will likely tell you
that there is no desktop manager present. That is only one theory, but it can
happen, if you are unfamiliar with having multiple desktops.

Have you considered installing Mint? It is easy to use and it uses Ubuntu's
repos. Assuming the answer is No, then try the ones below.

From the terminal you can try to login by typing: login. You can try to launch X
(the graphical interface) by typing: startx. You can reconfigure X by typing:
sudo dpkg-reconfigure xserver-xorg. It will ask you a bunch of questions, but
the defaults should get you working. I would try those things in order before
re-installing Ubuntu below.

You can use bash commands from the terminal to install missing pieces. Type:
sudo apt-get install ubuntu-desktop or sudo apt-get install gnome.
They will give you a working desktop. It will ask you for your password.

Let us know how you make out.