In Linux there are two commonly used boot managers -- the GRand Unified Bootloader (GRUB) and LILO. The way they work is that the Master Boot Record of your hard drive includes the information about how to boot the PC -- in other words, where to start. The first thing to happen after finding where to start is the boot loader runs. The boot loader typically provides you a menu of what you want to start -- Windows or Linux, in your case -- and then points the PC to the installation you choose.
If you install Linux side by side with a Windows machine, your Linux installation will probably recognize the Windows installation and will set up the boot loader and partitioning through a series of questions.
Here are some FAQs that might help you out on how to install a boot loader.
- FAQ on how to install WindowsXP and Linux with GRUB
- LILO FAQ at the Linux Documentation Project
- Multiboot with GRUB
Now the one thing that you may not have considered when setting up your installation is the ability to share documents back and forth from one operating system to another. The problem is that for your Windows installations Linux can read the FAT32 and NTFS file systems, but it can't reliably write to the NTFS. Windows XP and 2000 by default use the NTFS file system. My suggestion is that you create an additional partition either through the setup process of Linux or by using a partitioning tool like fdisk. Or you could use a Partition Magic-like partitioning clone under Linux called QTParted.
This partition should be of the FAT32 variety so that both Windows and Linux can write to it. This would be an ideal place to store documents you want to access from both operating systems.
For the Windows users I would use Microsoft's Power Toys to help tweak your Windows session. Instead of storing your Documents and Settings on your Windows NTFS driver, use the TweakUi tool.
Choose to move the location to the FAT32 partition under Windows. As you can see from the picture above, it's on my E:\ drive. Then under Linux you can create a link in your /home/$user/ directory to that same My Documents folder and save files from both operating systems in the same place. You can do this for both variations of Windows you need to install and make things more easily accessible from any of the three operating systems.