I have a PC that runs Windows XP Professional and Linux Red Hat 9.
Basically how I partitioned it:
I used Partition Magic (
http://www.powerquest.com/partitionmagic/)
Create 2 EXT3 partitions from the free space on my NTFS partition for Windows XP. One for the / and /boot mount points used for Linux.
Create 1 Swap partition from the remaining free space on my NTFS partition. (Must be sized twice the amount of total RAM in your PC)
Restart and let the changes take effect.
Restart again and install Linux to the newly created partitions.
Install the Grub boot loader during the Linux installation and make either Linux or Windows the primary OS. And you've got a fully operational dual-boot system.