Vista no longer uses the same boot loader and boot process as XP/2003, so if you plan on having XP and/or 2003 on your system, you should install Vista after installing XP and/or 2003. If you are in the unfortunate scenario of installing Vista first and then trying to get XP or 2003 to work, you can follow
this post to get it working.
If you install XP and/or 2003 before Vista, you should have no problems at all.