When selecting the drive you wish to install the OS on, simply select the external HDD.
Yes, if it's formatted to FAT or NTFS.
Yes. Any hard drive, flash drive, or other external storage device that conforms to the USB Mass Storage device standard will work on Windows Vista.
Of course, backup all your files to an external hard drive and perform a clean install with your Vista disk. They only make it easy to upgrade never downgrade. If you do not have an external hard drive, create a disk partition to use as your file backup.
There's no such thing as Windows XP format or Windows Vista format. You should check however if your external hard drive uses an NTFS or FAT32 partition table. You can see that by right-clicking the drive (C, D, E, ..) and clicking properties. If it uses NTFS it will be no problem for Vista. If it's FAT32, then google for a way to convert it to NTFS. No big deal.
You can certainly move Windows to an external hard drive but Windows will not boot directly from an external drive. If you are running Windows in Parallels (See links below) you can have Parallels installed on the Mac's drive and then have your Windows virtual machine on the external drive.
No. Vista ruins all windows(non-vista) and RAIDed partitions.
The hard drive stores the Operating System. Like for example : Windows XP or Windows Vista or even Windows 7 without the hard drive your computer will not turn on.
In our old computer I put a SEAGATE 500 gig External Hard Drive. The computer was windows XP Is it a working chance to use the external hard drive From The system i first installed it.
If it has the minimum RAM and hard drive space, you can.
Yes, you can. But there are some requirements for USB drives. More information you can find here http://www.pcstats.com/articleview.cfm?articleID=2160
Yes. The SATA internal hard drive in Vista can be turned off, by turning off the feature in system BIOS.
Yes you can, when the computer is booting up open the boot menu and choose the external as your boot device, i have a external hard drive with Ubuntu on it and do exactly that, I'm sure it can work with windows.