seta r_customheight "600"
seta r_customwidth "800"
Wrong
seta r_mode "3"
then run the game and use options menu to set any resolution you want
LOWER than 1600x1200. If the problem happens again with another resolution, repeat the procedure and remember that the resolution is not available for fullscreen OpenGL on your computer.
Also make sure you look for your base folder in Documents and Settings,
NOT Program Files.