Instead of using the .xinitrc file in your user's directory, try using it in your /etc/xinit directory. Open it using your favorite text editor and input the proper command to bring it up. Some examples:
exec gnome-session (I think--don't use gnome too often)
There are as many different commands to call up the different desktop environments and window managers as there are DE's and WM's. Do some searching to determine the correct ones. Put them all in your /etc/xinit/xinitrc directory and then comment out all but one at a time:
Then, go back to CLI and type in "startx". You'll bring up that X session. If you don't like it, logout of it and go back to your file and comment that one out, then uncomment the next one. Find the one you like the best and you can stick with it. BTW, the file name might be different for you as I don't run Mandrake, but Slackware. That file will boot ALL users into that specific window manager/desktop environment. If you don't want that, then edit your individual user accounts to reflect the WM/DE you want them to have. I hope this helps.