This is caused by Windows deleting the master boot record where grub resides, don't fret, your data is safe. but in order to get the installer back, you'll need to re-install grub. i personally use the super grub disk [link]http://freshmeat.net/projects/supergrub/[/link]
once you're in linux, log in as root and issue the command grub-install /dev/<HD>
replace <HD> with whatever your device is.
once you reboot, you should have your grub back