I think you have to restore Grub if you install Windows 7:
Assuming that you are installing
Windows 7 on a separate partition in a
machine that had Windows XP and Linux
installed already.
If your machine had only Windows XP
and no Linux, Windows 7 is generous
enough to let you choose to boot older
version of Windows through its boot
manager.
But, it deliberately ignore other
non-windows OS and file system, and
hence you can’t boot Linux through
Windows 7 boot manager.
In such case, you need to restore GRUB
boot manager. Follow these steps
carefully and restore your old GRUB:
Boot through Ubuntu Linux Live CD/DVD
Start terminal and give following
command:
$ sudo grub
You will then enter in grub
configuration mode. Here, give
following command:
$ root (hd0,0)
Then give following command:
$ setup (hd%1,%2) - here, %1,%2 is the
disk volume and partition value of
your original Linux installation where
/boot/grub is already installed. For
example, in my machine, it is on disk
1, partition 6, so the value will be –
(hd0,5). So, be careful here else your
Grub will not be restored, and you
will get error message instead.
Finally, give following command within
grub configuration mode:
$ quit
$ exit
Now, reboot (do not forget to remove
Linux Live CD/DVD to boot from HDD).
If everything had gone right, your
Grub boot manager will load and prompt
you to choose to boot from either
Linux or Windows 7. Enjoy!