Windows overwrote your GRUB install on the MBR of your hard drive. You simply need to rebuild grub. Some distros' install CDs come with a grub-rebuild function, which would be the easiest way to solve this. That said, doing it by hand is not that hard.
there are also many threads here on LQ about rebuilding grub. Essentially, you boot from a live CD (like Knoppix or Ubuntu), then, in a terminal, run "grub" then set the root & run setup, so... something like:
of course, you'll need to be sure the numbers are correct (i.e. hd0 or hd1 or hd2, etc). The link above has a good explanation.