Actually, dual booting is easy. Things only get complicated because of Windows, because it usually f*cks up the MBR and makes himself recognize as the ONLY operating system in your hard drive.
You can google "ntldr" and "bootmgr" for booting tips related to XP and Vista/7. Additional jargons: "boot.ini" and "bcdedit".
And another thing: Windows usually conquers the whole hard drive. Two options would be (1) reformatting the hard drive and install the 2 OSes; or (2) Shrink the volume, and create another primary partition for your second OS.