You have to edit your xorg.conf file. At a command prompt, issue this command (as root)
Look for where it says 'driver', you need to change this to 'vesa' (without the ticks on either side of it). Depending on your video card, it may show something like 'nv' or 'ati'. It needs to show vesa. Then, later after you've installed, you can go back and install the proprietary driver for your card.