Well make a large unpartitioned amount of space on your computer. Something like around 10gigs or so.
Eventually you need a minimum of 2 partitions just to run Linux. A root partition and a swap partition.
The installer will partition the extra space for you and format it. It will detect the WinXP install and should setup a boot loader that you can use to boot either into Linux or into WinXP. All of this will be taken care of mostly automaticly. Read carefully thru the install steps, if you can't figure something out or you don't know or understand then just accept the defaults.
If you want to make your own partitions, make one that will take up most of the free space, 10-20gigs is great. A full install can sometimes take up to 7gigs worth of disk space (full install is recommended for new users, so you don't accidently miss anything), note that includes everything from several media players, developement tools, office apps, and more. Then make a second partition between 500megs to 1gig, and that will be your swap partition. Linux has it's own file formats that are different from Windows. (The installer will take care of all this if you let it.)
Note that Windows can't deal with Linux formats, Linux can read and write to Fat32 partitions, but don't have a safe way to write to NTFS volumes. So that if you want to move files back and forth between Windows and Linux and your Windows is formatted in NTFS (very recommended, btw. Fat32 sucks) then make a small fat32 partition that you can "mount" to a directory in Linux.
Just so you know Linux doesn't use C: D: type drives like Windows/Dos does. It uses a directory tree and filing systems are mounted to empty directories. That way it's transparent to a user. The info could be on the root partition, and your home directory could be a entirely new directory or a network file system share on a completely different computer. Whatever you want.
Make sure that you have a working WinXP disk so that you can get into the recovery console and be able to restore the WinXP boot loader in case you decide that you don't want to use Linux.
After you get it installed check out the mandrake forums and read some introductionary documentition. If you check out my sig you will find a link to several guides. There is a linux introduction guide that will show you the basics of Linux. A lot of it is command line heavy and designed for people that want to develope administrative skills. There are lots and lots of other guides and websites designed for newbies with different ways and angles on explaining stuff. A search on google can find anything you want to know about anything.
There is even a linux specific search, goto
www.google.com/linux. (no trailing /).
After years and years of using Windows people often have a hard time translating their thinking to the Unix-way of doing things. So dont' worry if you find some things frustrating.