Booting vista from flash drive

Hello
I sell Sony Vaio Laptops which come pre-installed with vista premium. I'm looking for a solution to be able to demonstrate or show laptop working without the need of the initial setup so i can sell it as new without the need of deleting users or restoring to factory default etc.
One way I've thought of was to boot from a flash drive, however my concern is if using this method does it write anything to the hard drive for reasons as mentioned above.
Many thanks
Paul