I have reinstalled my Vista home premium 64bit, over 20 times in my computer due to hardware upgrades and atimes uninstalling it so that i can use XP to run some benches. It will auto reactivate online, if there is no hardware changed(adding extra rams or a new hard drive, woun´t affect it) but if new hardwares like a new cpu been added, you will only need to repair the os(vista) and the new drivers for the new cpu will be added it. The problem comes up when a the boot sector is freshly formatted and the vista reinstalled with maybe a cpu, motherboard and extra hd added, then one has to call ms to activate it. Which is equally easy to do.