Let's say you've just reinstalled Windows, and are up and running fine, only you can't boot your Ubuntu install anymore because Windows has wiped out GRUB, the bootloader needed to boot Ubuntu. If you're a seasoned Linux guru, you'll instantly whip out your collection of recovery disks and hammer out a few commands to restore GRUB; however, if you're simply an average user who hasn't memorized the commands required to restore GRUB, don't have the necessary liveCDs or can't boot them, or simply want to avoid the command line, this guide will help you easily restore GRUB.
A bootable Windows install; any version from 98 to Vista will work.
Note: An existing install of a modern Linux distro (Ubuntu, Fedora, etc) will also work (same instructions, just use the deb/rpm/sh package on the download page), though this guide focuses on Windows.
What this Does (technical details)
When you run the .exe file, it modifies your Windows bootloader to provide an option to load GRLDR from GRUB4DOS. GRLDR, in turn, loads a floppy image containing the Super Grub Disk bootloader repair utility via Memdisk. This way, it loads the floppy image into RAM and boots from it, allowing you to start up the GRUB recovery tool and repair/restore your GRUB install. Afterwards, upon the next Windows bootup, the utility uninstalls itself and removes the extra menu entry for loading the Super Grub Disk image from Windows, thus leaving you with only a standard dual-boot GRUB configuration between Windows and Ubuntu.
1. Download the latest Windows (.exe) version of UNetbootin
2. Run the .exe file, select "Super Grub Disk" in the distribution selector box and press OK, wait as Super Grub Disk is downloaded and installed, then reboot:
3. Upon the next bootup, you'll see 2 boot menu entries: One for Windows, and below it, one labeled UNetbootin-supergrubdisk.
Select the UNetbootin-supergrubdisk boot option, press enter, and it will present you with the Super Grub Disk interface.
4. Select the default options ("Super Grub Disk with Help" and "English Super Grub Disk") on the first 2 prompts, press enter a few times after that to accept the license terms and see the instructions, then you'll get to a page titled "English Super Grub Disk (Help)":
Select the first option, "GNU/Linux".
Press enter a few times again to scroll through the instructions, and you'll see a menu titled "GNU/Linux (Help)":
Select the first option, "Fix Boot of GNU/Linux (GRUB)". Now it will ask you to specify which Ubuntu install's GRUB bootloader to restore. Given that you likely only have 1 install, select the first among the options; it should have an entry under "OS" titled either "Boot" or "Ubuntu":
5. Now, you'll be presented with a success screen, and can now reboot. Do so, and upon the next reboot, the GRUB menu should now show up and you should again be able to dual-boot between Windows and Ubuntu.
However, the next time you boot Windows, should you want to remove the second menu entry generated by UNetbootin for loading the Super Grub Disk, simply select yes when prompted "Are you sure you want to remove UNetbootin?" after logging in to remove UNetbootin, and that'll undo the Windows bootloader changes so you're left with only GRUB and a standard dual-boot setup.
The Super Grub Disk can also be used for various other purposes; for a description of its more advanced options and functionality see the documentation on the site. UNetbootin, which also supports liveUSB creation and no-CD installations of Ubuntu and various other distros, was created by me, the other tools used (Super Grub Disk, GRUB4DOS, Memdisk) were created by their respective authors. Feel free to post a question if you have issues with the loader (UNetbootin) itself, but if it's a general Super Grub Disk bug, make sure to report it on the Super Grub Disk site.