Firstly - check that your Vista systems have all the drivers (Motherboard, Graphice, etc) for XP.
Secondly - buy your two copies of XP Pro - note - Office has nothing to do with the OS.
Thirdly - save all you work and downloaded programs.
Fourthly - download all the appropriate drivers
Fifthly - boot up with your XP Cds reformat and install XP.
Sixthly - Install drivers, software and data.
You can't boot XP from external drives.
It'll be cheaper to get XP Home, and you won't be missing anything of note.
Check with your system suppliers if they offer a low cost XP option for this circumstance.