No MS System I know can be installed without being able to boot immediately. Every Windows installer by default thinks it is the only important system of the PC and will overwrite the MBR without giving it a thought.
So let the bugger have it!
Grub2 can be re-installed/restored/replicated by just using any Linux Live CD that has Grub2 (say Ubuntu 10.04).
If you want to do it in a cunning way then before installing Win7 into sda2 add this lines to /boot/grub/grub.cfg to your Linux in sda1.
Code:
menuentry 'Win7 in sda2 known to Grub2 as (hd0,2)' {
set root=(hd0,2)
chainloader +1
}
You then install Win7 into sda2. I would recommend you alter the partition type of sda2 to Type 7 for NTFS so that Win7 installer will sink its teeth into it and won't let go. Thereby your other partition has no risk of being used by Win7 installer. You can alter a partition type by the standard facilities in cfdisk or fdisk.
Once Win7 has been installed and operating normally you can boot up a Linux Live CD and issue this command in a terminal to restore Grub2
Code:
sudo su
mkdir /mnt/sda1
mount /dev/sda1 /mnt/sda1
grub-install --root-directory=/mnt/sda1 /dev/sda
SInce you have already have the choice of booting Win7 in Grub2 (before you install Win7) then once Grub2 has been restored it will offer you Win7 as a booting choice.