I have one solution that may work for you. Boot up the Kubuntu CD as a Live CD.
1) Mount your drive read-write (should be something like /dev/hda1 and it should be mounted in /mnt/hda1)
2) Navigate on your hard drive (/dev/hda1) to find the hard drive's /etc/passwd (/mnt/hda1/etc/passwd) and open it (it should contain only the lines you didn't delete). You might have to be root.
3) open the Live CD's /etc/passwd (Navigate with Konqueror to the / directory then /etc). This file should have all of the system services and stuff that were present on your original install, but might be missing some of the ones you setup after that.
4) Copy the contents from /etc/passwd to /dev/hda1/etc/passwd but be careful not to overwrite the file or delete the lines you left (which should be your user).
5) Save the file /dev/hda1/etc/passwd and reboot into your install. You should now have all the other users back at least from the base install.
If you installed other programs, like apache2, you'll have to reinstall them sudo aptitude reinstall apache2
. Just be sure to save the config files you modified (copy them to to something like file.conf.new or something like that). That way you'll be sure they don't over write your changes and if it does, you can just copy the file back.
Now, you've learned a couple of important things here. The first is that you need to read a bit more about how linux works. You'll find that programs and services create users for themselves in linux that give them restricted permissions on what they can do. For example, ssh creates a user "ssh" so it has restricted permissions. On windows and ssh server will run as Adminstrator or System and have full access to everything. So a security flaw will compromise your system. On linux, the system may be partially compromised, but at least the hacker won't have full root access.
The second thing you should have learned from this is to always make a copy of system files before you start mucking with them. sudo cp filename.conf filename.conf.orig
is your best friend when you screwed up a file. I even do it for simple files in my /home folder (like .bash_profile and .bashrc).
Third, this might be one case where reinstallation isn't a bad thing. I know it took you a whole day to setup this time how you wanted it, but it's possible that you could do it a lot faster the 2nd time around. If this was your first ever linux install, reinstalling might even be fun. So consider that if you think this is too difficult.
Anyway, welcome to Linux! With great power comes great responsibility. I'd take some time once you're up and running again to read some of the newbie docs around the web. If you search around these forums, you'll find some other helpful links. But here's a few:
Good luck and hope this helps! Come back if you need more help.