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.