I had to install an HP HDX18 laptop for a friend. It ships with Vista and two internal disks. The challenge was to get rid of Vista, install Windows 7 on it, keep system and programs on the first disk and user’s documents on the other disk.
I found a really nice tutorial on some Win7 forum that explains how to do it. This will just sums up the directions to achieve it but one really should read the whole document.