okay 1) you need to change the setting so that mandrake doesnt automatically log you in... it sohuld show a login screen ...
2) you go to failsafe mode
3) type afterstep or blackbox or whatever WM you want to run..
4) Ta-da!! the WM is running.. thers also ways to add it to your login menu next to gnome and kde and stuff... but you can always run stuff through failsafe.. those other things are just shortcuts but incase your wondering.. its in .xinitrc or .xclients or something like that.. im not sure [ i think one of them is for text and one for graphical... but i just run it from failsafe mode.. [which is a terminal...]! --hope i helped!!

good luck man!!