root user should always exist by default.
If you installed over PXE it's likely that you used a kickstart file.
If that's, true the kickstart file could have the
root password in cleartext (this is a bad practice, but still possible). You should be able to find where the kickstart file is located by checking the files in
If you have the
root password you can simply log into the
root account. On the GUI login you can usually select "other user" and enter the username
root, then enter the password.
Alternatively you can switch TTYS and login (press
F2). You'll get a text based login screen, just enter
root as the username and enter the password. When you're finished
F7 should usually take you back to your graphical screen.
As MichaelKjörling mentioned, some Linux distros restrict access to the
root account. This can mean you'll be forced to use a different TTY or prevent this method completely.
If the root password isn't available you can try booting into single-user, assuming
grub doesn't have a password configured (or you know the
When you reach the
grub menu you should be able to press
e to edit a line. If grub has a password it will prompt you here. Add
single to the end of the
grub command line and boot it (normally
x I believe) and you should get a
root terminal where you can add a user and change passwords.
If you boot a live distro on your machine, mount your local drive and chroot to it you'll have a root terminal that's essentially within your filesystem, and you can add and modify users like you normally would.
If none of these work for you, you'll probably have to reinstall.