Q #1

I have tried to change my
file grub/menu.lst. but
after I changed it and rebooted,
the compute halted in the booting,
just show

GRUB

on my screen.

I then use the installation CD to go
into rescue mode and recover the
original menu.lst from the menu.lst~.
but the same error message show up.

what can I do?

Q #2
I have two hard drives, say, A and B.
First I install windows on B when I
take the disk as primay master.
Then I install FC7 on A when take this
disk as primary master (without B).

Now I place A as the primary master
disk and B as the primary slave. I
want the grub menu of FC7 on A show
the option of Windows XP on B. What
can I do?

Thanks.

Since I suspect your 2 questions are related, I recommend reviewing some basics.

First, GRUB has to be installed knowing where it's "home base" (/boot directory) is. This gets "hard-coded" into the boot code that goes in the MBR--or sometimes in a boot sector. Suppose that you tell the grub installer that /boot is in the second hard disk, 1st partition. If you then do anything that changes the order, then you have to re-install grub.

For a bit more, look at the booting link below.

Can you now boot into your Linux install? If not, can you boot from a "live CD"? Somehow, we need to see the output of "fdisk -l"