You should backup NTDETECT.COM, ntldr, boot.ini under C drive. Try to access your internal hard disk after boot from your external drive. If you can access it, it should be good.
Try to get another installation disk (like XP), install it, it will fix/override the current ntldr and on restart, you can pick your old OS to start and it should work.
Try to get another installation disk (like XP), install it, it will fix/override the current ntldr and on restart, you can pick your old OS to start and it should work.