You want to make a Bootable external Clone of your HD for emergency in case of HD failure. Actually I don't think the clone is the best choice, why don't you just backup whole hard disk to an image and save the image on your external hard drive, then you can have a recovery when facing with HD failure. Backup costs less hard disk space than clone. Of course, if you insist to clone HD, it's ok. Here I recommend you a free backup software which works well with Windows XP. EaseUS Todo Backup Free (http://www.todo-backup.com/)
, it supports file backup (backup single files and folders,) disk/partition backup (backup whole HD or partition) and system backup (one click to backup system) and etc. It supports hard disk clone and partition clone. free download to have a try.