There are a few different ways; I will go from easiest to hardest and let you choose.
1. Boot from a Knoppix cd or any other LiveCD that loads linux that way. Then mount your Fedora root partition and chroot into it (chroot /mnt/fedora where /mnt/fedora is where you mounted yoru Fedora partition). This puts you into your Fedora system. Then run grub again and type this: root (hdx,y) where x is the drive number (0 based) and y is the partition number (again, 0 based). So if Fedora is on /dev/hdb3, then it would look like root(hd1,2). Then type setup (hd0) to install it in the master bootrecord. Then type quit. You may also have to edit your grub.conf file to add lines to boot Windows.
2. Use CD 1 from most any Linux distro. This will load their installer, but you can switch to a different terminal (Ctrl + Alt F2 for example) and run similar commands to the above.
3. Use the Windows boot manager. In this case, you still need to access grub, as you have to feed it the first 1024 bytes of your grub.conf I believe. It's been a long time since I tried this way, so I could be way off on that (but I know it needs part of a file from linux).
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