factory reset
Hi, Vista should have the option to do a factory reset either in the control panel or in the folder where it is installed. You can also try to do the factory resetting using the command prompt as someone has already advised you. A factory reset will not uninstall Vista but may uninstall all you programs or software updates for any added hardware and you just would have to reinstall them. Make a backup of all your personal files too, as they may get deletes too.
good luck,
stewie