Problems of Second writing of MBR with GRUB

[L V Gandhi <lvglist@gmail.com>]

I have already installed windows xp and squeezei386 with grub in MBR.
I would like install squeeze amd64 in other partition for multi boot.
If I install grub in MBR while installing squeezeamd64, then how do I
boot squeezei386 as its boot info is  not in its partition boot
sector.
How should I go about to boot all the three OSes.
-- 
L V Gandhi