How to unlock secret characters without beating single player:
In your system folder is the user.ini file. Just open it with Notepad or something similar. You'll notice that most of commands are set out like this. [Factor] Text and Variables of that factor
There is always a gap between the commands. Make a new gap in the command lines and paste this in there.
Then you'll have all the secret characters to pick on the character select screen. This is good to use if you accidentally ruin your user.ini file and it gets reset.
How to increase resolution and give widescreen support:
Simply edit the ut2004.ini file in the system folder, look for the section below and edit accordingly:
Also, you may want to edit the user.ini file and change the FOV (field of view) to 106.270273206 since it is now in widescreen. Should say something like: DesiredFOV=xx.xxxxxx DefaultFOV=xx.xxxxxx