I'm pretty sure you needed to use a tool that comes with Vista, BCDEDIT, to remove the Vista bootloader from the boot sector of the hard disk and replace it with whatever other OS you want to start. Since you already removed Vista, maybe the best thing to do is use the XP recovery console and use FIXMBR and FIXBOOT to restore XPs boot loader. I'm making the assumption that's what you want to do, but if its some other OS then do whatever is appropriate to restore that system's boot sector.