I have a pc with windows vista and I bought and external harddrive to install windows xp?

please can you tell me step by step how to do the installation?

I have a pc with windows vista and I bought and external harddrive to install windows xp?free anti virus

cant. XP has to be on an internal drive. this is because it reboots all USB devices on start up. you can have them BOTH on the internal drive however.

I have a pc with windows vista and I bought and external harddrive to install windows xp?internet security software

http://support.microsoft.com/kb/927817

http://support.microsoft.com/kb/307844
if you got your computer with vista on it then i wouldn't down grade xp will not be able to power some of the updated hardware and your computer will not work right. i know because i tried
right, look up how to dual boot on google. you dont need another hardrive though. you get the choice of what Operatng system you want to use on boot up.

you can just make a partition .

i would of thought you could of changed the bios so that it boots from the external hardrive and not the internal one. and install vista on that . then when you want to use xp change where to boot from first. change it to try and boot from the internal hardrive.

should be in the setting where you choose where the pc boots from first e.g floppy, cd drive...

confusing but i no what i mean. hope you do
You see, external hard drives have been built with a special physical format which is not compatible with running operating systems but actually just storing the data from a working OS. What I mean is that you can hold operating system files but cannot run them as an individual real time application whereas movies, music, photos etc. are stored in their but run by applications on your current OS. So it unfortunately is not possible. Sorry.

Hope I was of help.
i dont think windows xp can override vista ,stick with vista its great dude