the installer would (should) have changed the MBR (master boot record) on your master hard disk to put a grub there from the / partion.
if you reinstall windows it will overwrite the MBR and you will get no boot loader. but you can restore the MBR using the live CD and using the grub commad.
you need to know where grub is. i expect it is in the second partition on your master disk hd0,1 or sda1.
in a terminal run:
# grub
# root (hd0,1)
# setup (hd0)
# quit
reboot and hopefully that fixed it.
