Firstly, I assume that the copy of XP you want to install is not installed on another machine already?
If so, you MAY be able to put the CD the laptop, boot from CD and start the XP install routine, choosing to install to the same disk as Vista & opting to format the disk. However, Vista may not allow you to even get this far. Windows installations don't seem to like earlier versions for some reason...
You may need to look on the net for some 3rd party software that can recognise and format NTFS drives from a bootable CD. There's a few out there.
Now onto your second question. Changing back to VISTA later....that could be another story. Is the disk that came with the laptop an actual copy of Vista or is it a 'recovery CD'? If it's a recovery CD, it's possible that your Vista install files are either a) on a hidden partition on the laptop, b) on a visible partition or c) on the C: drive althuogh this is unlikely.
The Recovery CD would format the drive and restore the laptop to the state it was on delivery with Vista running happily no doubt. However, formatting your C: drive to put XP on COULD wipe information needed by the recovery program. If your CD is a copy of Vista, then it doesn't matter. YOu can just use that to reinstall.
I wouldn't rush at it anyway. Take some time to fully learn what you've got in the box, read the instructions - it should outline recovery situations in there somewhere and check you have EVERYTHING that would be needed to get the system back.
May I ask why you/she wants XP instead of Vista? It seems a big improvment over XP so far. (As an aside, what version of Vista is installed? I've read of some new PCs only shipping with Home Basic).
If it's an appearance issue then she should get used to it in time. If it's program compatability problems, maybe this forum can help with some workarounds.
First and foremost don't rush into anything. You could end up with a laptop that has XP on it with no option to restore Vista later. Don't forget you've effectively paid for Vista so it would be a shame to lose it.
You may want to look here: click here (first post there) for how hard it can be to remove Vista.