Your machine is having trouble finding an appropriate mode (size and resolution) for your monitor. You can restart X by opening up a console and typing init 3 as root. Then restart x with startx or init 5. I'm quite sure this will work, at least on Mandrake Linux.
Also edit the aforementioned file /etc/X11/XF86Config to pick a less intensive (less res, size) video mode if things don't work out.