hmmmm... could it be that your monitor or video card has been setup wrong?
If mandrake automatically setup your video, this is possible. It is also easy to fix from the command line.
If you boot up off your mandrake CD, go into rescue mode by pressing F1 at the splash and typing rescue.
After you get into there i believe you can type "setup" then a menu will come up, choose X configuration and it will ask you what video card, monitor you have. It will update your X configuration file with the new info about your video card and monitor.
I'm pretty sure you should be able to do this in rescue mode. I've used linux off and on (like you) and have frequented using it recently.