You can grab the kernel source from http://kernel.org
, I suggest you locate the same version you are running atm. They have every single version stashed in their ftp. Just unpack it under /usr/src/linux-<version>
You could also try to locate a mandrake kernel source package, there surely is one.. (:
Im not sure how the ndiswrapper is installed, but if it needs a _kernel_ recompile, then I suggest you do a forum search here with "kernel compile" and youll surely find help on that. If it just needs the kernel sources for its own compile, then you can just go as the INSTALL or README for it tells you to.
And the last one. Why would you want to login as root? The newer versions of kde have it disabled for a reason, it isnt a good thing to be actually logged in graphically as root. Does mandrake disable normal virtual consoles (ctrl+alt+F2 etc) when in the graphical runlevel? If it does, you can allways lower your default runlevel by one step. I have no idea how its done in mdk, but most distributions have it in /etc/inittab