Actually, this is quite straight forward.
When you "fire up" the mandrake install disc, it will ask you where you want to put it, you should see the options already mentioned. You can then (probably using a drop down) tell it to put it on the empty partition (if you know the size then brilliant, because mandrake might not call it "D", but 18gig NTFS or something like that), then you know it's going to the right place.
By default, mandrake uses Lilo for the bootloader, yes it will overwrite the windows bootloader, you must tell it to install to the MBR. Don't worry about that, because in the unlikely event that your XP won't boot, you just put the mandrake install disc (the first one if you're using cd's) back in, boot it, when it asks you about install or more options, hit F1, and then it gives you a command prompt, you type in rescue, and then when you see the options box, you just choose "re-install windows bootloader".
For a desktop, mandrake also defaults to KDE, so when the install progresses choose kde, you'll get all the Qt libs etc etc (at a later stage, when things are "up and running", you might want to install the GTK libs as well (you can do that without having to install gnome), because there are some app's that use them.
Ha, I found all this stuff out after I'd said F*** it, and just thrown the disc in and told it to install and just accepted all the defaults. My system, originally, had 3 partitions, the C, with the windows on it, D, that was just called "Data", formatted but otherwise empty and E, was called Recovery, only 2 gig's in size so of no use.
Hell, if you've backed up your data, and you have access to a windows recovery type disc, then just go for it, worst case scenario (but quite unlikely) is that you'd have to re-install that as well.
Go for it my friend, mandrake is a good distro to start with, I found it very easy to use, helpful, and there's absolutely mountains of support/help out there!
Some other excellent links for mandrake are
(which is good, but sometimes you can't access the site).
which I found brilliant. You just follow the instructions, and select a mirror source for all the sections and then just "Cut 'n Paste" the resulting command(s) into a root terminal. Then when you go into the RPMDrake package manager to try other software, you can just click a box and hit install - My favourite was the stuff provided by the "PLF", just that much better/prettier/nicer to look at with a few extra bits.
One good thing for you to look into later, would IMO be extra hard disc space. Why? Because although for my first year of linux, I happily ran mandrake, but the downside of having everything on the one partition, is that if/when they release a newer version, if you havent' backed up your personal data, when you install it over the top (I never did successfully manage to do an "update/upgrade" of the whole distro), you loose anything that you might want to keep. The easiest way to get round that is to have a seperate /home partition, so when you install a new (or different) version, you just install it to the /root partition and don't format/touch the /home one.
As I say, that's something for later though. Just get it installed
I can't enthuse about it enough. It (linux, but mandrake specifically) it very good, easy to manage and IMO very usable (a lot moreso than some distro's that I've tried!)