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Well, your error message is not a very pretty one. I've never seen it before, but I believe this is caused because either you or Mandrake, during the install process, had got wrong information about your hardware, either video card or monitor. There has to be a command line tool for mandrake to reprobe the monitor/video card, so you can change the setting before typing startx. Maybe someone here with more Mandrake experience will show you which tool that is.... For example, with Redhat, we type redhat-config-xfree, to re-setup the video properties.
Indeed, you could reinstall the hole thing again and make sure that Mandrake installer probes correctly everything on your machine, specially video, monitor and mouse to avoid pos-install issues.
Some files which are good for you to check is /etc/X11/XF86Config.
If you read the "almost end" of that file carefully, you might try to edit the configurations manually. You will need to know how to handle a command line text editor as pico (type pico + name of the file to open, you also have to be root). Another thing to check is /var/log/XFree86.log. That file contains the informations in details about what happened when you've typed startx and possibly why it has failed...
If you are totally new to Linux, I recommend you reinstalling the distro, making sure that it probes everything correctly (it tests the video during the install, make sure it works this time). An average Linux user might fix the problem, but right at the beginning trying to fix a video problem, learn a text editor, verify files, paths and other stuff can be very frustrating. It's much easier to have a 100% working system and brake it while learning... my two cents!.
Good luck my friend!