For dual booting, you should read this post: http://www.linuxquestions.org/questi...hreadid=196636
and perhaps take a look at others related to setting up dual boot. If you are running lilo, you need to add an entry for Windows XP partition in the file /etc/lilo.conf, then run the command "lilo" as root to get changes to take effect.
To run cfdisk, you need to open a terminal enter "su" to become root, then enter the command "cfdisk".
It probably isn't a matter of having a hidden partition, but having the boot loader installed and configured properly.
In Mandrake starting out, it is best to try and do as much as possible from the Mandrake Control Center. Take some time to learn all the options in there. There are some things you can do to configure your boot loader in the control center, but I don't have Mandrake in front of me right now. Check to see if you can add your windows partition from the Mandrake Control Center. Also, make sure the boot loader is installed in the master boot record.