Hi
Is this a computer with Vista preinstalled?
If so, I think it's a bad idea to try creating a "true" dual boot with XP.
Installing XP after Vista will make Vista unbootable - this can be fixed, but you'll need a Vista installation disc (NOT a recovery disc).
A better solution would be to get a second HDD and install XP there - then use BIOS to choose OS (or maybe edit XP's boot.ini).
And, yes, you can save files to both OS's partitions (also FAT32).