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.
http://h8ersclub.com/fix-grub-after-...all-pclinuxos/
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:
Code:
%> grub
grub> root(hd0,1)
grub> setup(hd0)
grub> exit
%> reboot
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.