I put the hard disk with Vista on it into a caddy and connected to a USB port if I could get the desktop to boot from there?
1. No windows will not boot from a USB drive without a lot of work
2. the drivers in windows VISTa for your laptop will not be right for your desktop and it will not boot
3. you would also be breaking the windows license agreement using your laptops copy of vista on your desktop unless it is a retail version and it would also require reactivating.