Well, i use RedHat 9.....but one thing i can say is that whatever the flavour......the names of packages are similar...
So in RedHat 9 , there's a tool called "Desktop Switching Tool" which lets you select your default Window Environment.
It will be located in the Mandrake Menu. Check out all the menus ....You should be able to locate either the same name or something matching to the above mentioned tool.
Since I havent used Mandrake in recent times, i cant tell u about that exactly.....