I've managed to get Fedora to install but whenever it starts up the GUI the image flickers horizontally accross the screen making it impossible to read anything. I had to run the installation in text mode to avoid this with the installation. This flickering is caused by linux displaying the GUI at a higher resolution then supported by the monitor (the monitor is old).

I have two questions:
1) How can I edit the screen resoloution settings in this incident?
2) Is there a text mode for fedora which I can boot into?

You can edit /etc/X11/xorg.conf if you are using x.org and change the resolutions for your selected color depth. Also, make sure you have the correct refresh rates for your monitor.

To get out of X, press ctrl+alt+backspace and it should take you to the console.

I tried the Ctrl + Alt + Backspace key combo but the command line login just flashes on the screen for a second or two then the GUI loads up again. Also, how do I edit files from the command line? (I'm fairly new to linux)

may be you can mount your files/partition in read-write mode using some rescue CD and then edit them.... (say /etc/X11/xorg.conf, etc)
Also check out /etc/inittab file for setting default runlevel to text mode or to graphical login.
No idea about flickering monitor could that be driver problem or incompatible hardware issues :-/ I have never faced such a thing before (thankfully !!)
--
regards

Fedora gives you a default runlevel of 5 (I think)
which tells the X server (gui) to startup
and gives you the graphical log-in at boot.
You can change this to something like runlevel 3 (maybe).
In this runlevel the x server is not started
and you get a console (text-mode) log in.

You need to search around for information on different ways
to change your runlevel, or more specifically your 'default runlevel'

Whether or not you can fix your 'flickering screen' problem
from there or not, I don't know, but
you can at least use your machine without getting a headache.


There are a number of text editors available for linux
that will run in a terminal or on console.
The choice is rather a matter of near religious debate.
I am comfortable with emacs, so I use it.
The learning curve can be a bit steep.
You might need a 'cheat-sheet' for some basic things
until it becomes natural for you.


If you just want to make changes on the command line without
opeing up an editor then you'll have to wait for
someone else's advice. I would only lead you astray.


Good luck!