To make along story short, the desktop PC won’t boot.

After some digging around, something must have happened to the BIOS? because all the reference points in the grub menu (/boot/grub/menu.lst) were pointing to (hd0,4).
But it turns out that (hd0,4) is not a partition all of a sudden, and it should be (hd0,3). I’m referring to /dev/sda4 partition.
Anyway, to install the grub menu in the mbr, try this in the grub menu console:
grub> root (hd0,3) #this I think tells grub which partition is root
grub> find /boot/grub/stage1 #this I think tells grub where to get the stage1 data.
grub> setup (hd0) #this I think tells grub where to install itself