Follow the Ubuntu Community instructions on restoring or recovering from a boot-loader problem. Essentially, you'll have to chroot into your hard drive environment using your live CD and use grub-install to reinstall GRUB. There are multiple ways to do this as suggested by the document - choose your pick.
from this earlier answerthis earlier answer.
You can find GRUB's menu at /boot/grub/menu.lst on Ubuntu systems.
If you just installed Windows, it's normal for it to overwrite the MBR. You might get the same thing if you just did an automated repair or recovery on your Windows partition. If this isn't a fresh install and you didn't just repair the system, something else is going on -- Windows doesn't usually "fix" the MBR for no reason.
