Grub error 21: Please help recovery the PC!

Question

I have been using my PC with two OSs (XP Pro and Ubuntu). Everything went wrong when I thought it was a good idea to install another version of Ubuntu to an external hard drive. When PC is turned on, the loading process fails at:

GRUB Loading Stage1.5.

GRUB loading, please wait... Error 21

It looks as if Bootloader (GRUB) is not found. Reading materials on the Internet and remembering vaguely what I have done, I think I missed two important steps to follow:

1. I did not select the external Hard drive as the location for installing GRUB.
2. Before restarting the computer, I did not edit Ubuntu part of /boot/grub/menu.lst (from (hd1,0) to (hd0,0); I am just mouthing second-hand information here. I know I did not do this but I don't know with what effect....). The article I read says I should have done this because I was installing 8.04, or else I will get grub error17. It also says that GRUB2 for 9.10 and higher don't have this problem (again, mouthing second-hand information).

Currently, the PC doesn't boot any OSs whether the external HDD is connected or not. When no external drive is connected, the process ends at Error 21 as above. When the external HDD is connected via USB, the screen has only one flickering underbar (BIOS cursor?) on the left top corner of the screen.

Here is how my partitions look like. If anyone can look at this and help me recover my PC without doing fresh installs, I really appreciate it.

ubuntu@ubuntu:~$ sudo fdisk -l

Disk /dev/sda: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0xcccdcccd

Device Boot Start End Blocks Id System
/dev/sda1 * 1 2611 20972826 7 HPFS/NTFS
/dev/sda2 9062 9729 5360040 12 Compaq diagnostics
Partition 2 does not end on cylinder boundary.
/dev/sda3 2612 5222 20971520+ 83 Linux
/dev/sda4 5222 9061 30836768 5 Extended
/dev/sda5 5223 8931 29792542+ 83 Linux
/dev/sda6 8932 9061 1044193+ 82 Linux swap / Solaris

Partition table entries are not in disk order

Disk /dev/sdb: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x000a2f94

Device Boot Start End Blocks Id System
/dev/sdb1 1 19273 154810341 83 Linux
/dev/sdb2 19274 19457 1477980 5 Extended
/dev/sdb5 19274 19457 1477948+ 82 Linux swap / Solaris 

Thank you very much in advance!