GRUB reported "error: file not found." when booting CentOS 5

Question

I installed CentOS 5.3 by using a 4GB USB disk on an old PC that I used run a Debian on it. After a series of housework on the fresh system, I reboot the system using:

sudo shutdown -r now

After that, the machine did not reboot successfully instead of directly powering down.

When I started the machine, after the BIOS, then GRUB.... But the GRUB gave this:

GRUB loading.
Welcome to GRUB!

error: file not found.
Entering rescue mode...
grub rescue>

Then I googled around and did find some information on this. As the post said, I did the following in an attempt to wake up the machine.

First, I entered:

ls

then I received:

(hd0) (hd0,msdos5) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1) (hd1) (hd1,msdos1)

and the last two partitions seems to be the ones in the USB disk.

Second, I entered:

ls (hd0)/

and received:

error: unknown filesystem.

I ls-ed the rest and get:

ls (hd0,msdos3)/
./ ../ lost+found/ home/ boot/ sys/ proc/ var/ tmp/ dev/ etc/ root/ selinux/ usr/ bin/ lib/ media/ mnt/ opt/ sbin/ srv/ misc/

and

ls (hd0,msdos1)/
./ ../ lost+found/ grub/ memtest86+-1.65 message initrd-2.6.18-128.el5.img System.map-2.6.18.el5 config-2.6.18-128.el5 symvers-2.6.18-128.el5.gz vmlinuz-2.6.18-128.el5

Then I followed the advice exactly on the very post to do this:

set prefix=(hd0,msdos1)/grub

and

set root=(hd0,msdos3)

After the settings, I entered:

insmod normal

and got:

error: file not found.

Could any one help me? I'm stucked....