Windows OS on External HDD?

it's possible and i see that the friends gave the best answers , so go ahead dude ;)

I think its possible. You just need to set your Boot Order to "External Hard Disk/External Drive" by going into Boot Setup.

Check out this link. You might find it very useful: http://www.ngine.de/article/id/8

by all mans one can install windows xp on external harddrive. you can even install it on your pen drive for that you need xp portable version. so install and enjoy.

Yes its very possible and tried that to my customer's asus eee4pc laptop. It is really working and I am using a tiny xp and can be download in minnova.org using bitcomet or any torrent downloader. But be careful, its better not to unplug/plug the usb hdd wire always because during my experience using tiny xp, the external hard drive can get easily make bsod error. I guess its an hard drive blue screen error.