no, it wont be that easy to install XP, the file system in Vista and XP differs. you need to wipe clean the H/D to get XP on.
Darik's Boot and Nuke (Hard Drive Disk Wipe)
download this free utility to clean the hard drive (fairly easy process), then you will be able to install XP, else keep trying, it wont install XP.
You don't need a program to install vista, just pop in the xp disk and reformat.