You want to create another cd containing GParted (gnu partition editor). You can resize your existing partitions (as long as you have 15g free space), and create the required parts. Just be careful, during the Ubuntu install there will be a question as to how you want to partition, do it manually and select the partitions you created previously. You can even create a mountpoint (directory where you can access your Windows partition/files from within Ubuntu), from that dialog.
It's not difficult, just be careful to read the prompts carefully before you click through.
Good luck.
(Just an FYI, you want to create two partitions in addition to your Fat32/NTFS, a linux native (preferably EXT2 or EXT3) and a swap partition which should be twice as large as how much RAM your system has - 512Mb of RAM, Swap Partition of 1Gb).