It looks like (somehow) grub is not installed.
I would suggest that you follow Zmyrgels instructions but install grub from an RPM before issuing the "grub" command. (as that wouldn't work otherwise
If you have the CD you used to install it should be on that, if not you'll need to download it.
use "rmp -i /path/to/grub.rpm" then follow the commands above (Zmyrgels post)
if your linux partition is the 2nd primary partition then the root section would read "root (hd0,1)" as grub numbers the partitions from 0. Then it should work. As a last resort before a compleat reinstall you could see if your install CD has an upgrade option, or install without a format.