If you are using two OS in same machine the most dangerous thing is reinstall one of them.
Whenever you install MS Windows then you no longer boots as Grub menu, you can’t boot your linux.
You don’t need to spend your time with dozens of unix commands to repair your grub now. There is a boot cd image which named SuperGrubDisk. It automatically finds your linux & windows partiton.
You can download it from : http://forjamari.linex.org/frs/download.php/1119/super_grub_disk_0.9766.iso