I think I know what you are talking about.
I've done this in the past. Not on a laptop but on a desktop that was running windows XP. I used Partition Magic 8.01, really any version. But I first resized the partition, made it smaller for XP, still leave some space for downloads.
In my case I had a 20gb hard drive. I resized it to 10 and 10 or whatever the exact was. Then restart and I had 2 drives. Then download whatever distribution of linux you want:
http://www.linuxiso.org/ . Or if you have your own cd already, make sure the bios is set to boot from cd, restart. Go into the installation, it will prompt you where you want to install linux, choose the new drive. Done.
I have did this with Mandrake 10.0, Fedora Project - Fedora Core 2. They both prompt you and have great GUI (Graphical User Interface)
Next one I'm going with, on seperate machine, solely for linux is either Debian, or FreeBSD.
That should be enough of a guide for you.
Enjoy,
Clay
Sr. Techncian
I-55 Internet Services
cwager@i-55.com