Use a live CD and chroot into your environment. If the rescue CD doesn't do this for you (and it should) you will need to tell it to (or use a different live CD, such as knoppix).
You will be required to know about your partitions. You can find some of this information by the command "fdisk -l". Identify where your Windows partition is, your "/" partition, and your "/boot" partition (if it is different than your "/" partition. In most cases it isn't).
You can see which partitions are mounted where by using the command "mount" on it's own. Once you have located where your "/" partition is currently mounted, go to that location. If it is not mounted, then mount it and go there.
mount --bind /dev ./dev
mount --bind /proc ./proc
From here, it will probably be a simple matter of:
There may be complications in this procedure. If you have a separate /boot partition, you will likely need to mount it (I think you can do this after you chroot with "mount /boot", but you might need to mount it before, or something). Of course, the biggest complication is the thing that I happen to be forgetting
Above all, good luck, and post back.