This is one of the most common questions here. Search on "dual-boot" or just "boot", and you'll get more than you wanted to know...
What is your configuration? # and size of hard drives, etc.
My ideal setup: 1 drive for all of the OSes and their applications. #2 drive--FAT32--for data.
The standard drill:
1. install windows on a 15-20GB partition. Leave the rest of the disk(s) unpartitioned.
2. install Linux #1:
During install, give linux a total of ~15GB--leave the rest unpartitioned
When it comes to installing the bootloader, put it on a floppy. This way, Windows will still boot if you have a problem. You can alway put the boot loader on the mbr later.
3. Once the system is working correctly with Windows + Linux #2, then install #2:
Do not install a boot loader
After install, reboot into Linux #1
Mount the partition containing #2, and go into /boot and find the name of #2's kernel and initrd.
Go back to /boot for #1 and find the bootloader config file. (eg for grub, it is usually /boot/grub/menu.lst) Add an entry to this file to point to Linux #2. The details of this will depend on your configuration.
The best approach will be to get Windows + 1 Linux working, and then study a bit to be sure you know how the bootloader configuration file is set up. Then, changing it for #2 will be easy.