The Windows 7 Installation disk has an option to recover "lost" bootable partitions. In my case I got a new hard drive and set up 64-bit Win 7 on it. Doing that, however, made my original 32-bit Win 7 boot drive (which was on a separate physical drive) invisible. Prior to my new hard drive, I only had the original single boot drive, so I needed to (a) find and recover the lost (original) boot drive, and (b) set up a dual boot menu.
"Repair" does all of those things. The function is buried a few menus deep in the install process which can be very confusing if you've never done it before, so here are detailed instructions which I found in another forum:
1. Beginning state: One bootable partition, but a spare bootable partition or drive that is currently not accessable on boot-up.
(Note: This happens a lot when you get a new hard drive and want to put a new OS on it yet want to save your old bootable drive to boot from, usually to store and access some software that may be incompatible with the new OS).
2. Insert the Windows installation disk (not sure, but either a 64-bit or 32-bit DVD would likely work equally well), and use whatever method your computer requires to boot from the DVD rather than HDD.
(Note: this is usually selected either from the BIOS setup screen or sometimes a good BIOs will provide the option on boot up, usually after pressing a special key to flag it into the mode where you can pick a boot-up device).
3. In the first screen after boot-up from the DVD, select the appropriate location and keyboard input, press "Next".
4. Since this is an installation disk, the next screen has a large "Install Now" prompt. Do not select that. Instead, select the "Repair your computer" printed in smaller text at the lower left of the screen. It can be easy to miss if you are not familiar with what the intallation disk can do.
5. The System Repair tool searches for valid operating systems and will state, "Windows found problems with your computer's startup options. Do you want to apply repairs and restart your computer?" Click "Repair and restart".
6. On the next reboot a B/W command level screen will be titled, "Windows Boot Manager". It should display the new bootable partition and the newly found old boot partition as two different boot options. In my case, it displayed the 64-bit boot option first, the 32-bit option next:
Windows 7 Home Premium (recovered)"
7. This screen is now a permanent fixture in the boot sequence and the installation DVD can be removed.
(Note: Thereafter, every time you boot, the boot manager will appear to give you a choice of your two partitions to boot from. I suspect more than two partitions will work just as well, but you may have to reboot several times, possibly once per partition to recover or include them all. Also if there are other fixable boot issues, the "repair" may also require more than one reboot. However, for a non-damaged boot drive that is fully intact, just "lost", I found it only took one reboot to both find the lost boot drive and mount it in a dual boot menu.)