Mandrake is a champ for newbs -- although I don't consider myself a newb any more, I still use Mandrake in preference to all other distros (currently running MDK10.0). It's very desktop-oriented, which makes it the perfect option for people who prefer wizards and GUIs to set up/maintain their system, rather than consoles and direct config file editing (as Slack does.)
You'll find that Mandrake is so easy to install you could do it in a drunken stupor. If you're planning to install Mandrake 9.0, 9.1, 9.2 or 10.0, all of them have a sweet graphical interface that'll walk you through every step. Once installed, the system should be all ready to go -- if not, the Mandrake Control Centre will let you tweak hardware, video, network, etc., through a nice GUI. Not much harder than in Windows -- in my experience, easier than Windows
About your harddrive: the Mandrake Installer is very smart when it comes to handling harddrives with other OSs or partitions already on them. You'll be given a number of options as to what you want to do, and if none of them suit your needs you can select advance mode. This will give you a graphical overview of your harddrive, and it's a sinch to cut out a new partition or use an existing one for installation.
I suggest your direct the Mandrake Installer to use your current Slackware partitions. It'll simply reformat them (ie, scrub Slack off your HDD) then do a new install of Mandrake in its place.
Phew. I sound like the Mandrake Fan Club
Tim