Can't boot Windows 8 after installing Ubuntu

I have Ubuntu configured to dual boot on my Windows 8 laptop. When I first installed it, I partitioned ~100GB for Ubuntu and left the rest to Windows (I might've messed up the partitioning.). Now when I try to boot Windows 8, the Windows Boot Manager tells me it has failed.

Specifically:

Windows Boot Manager  

Windows failed to start. A recent hardware or software change might be the cause.  
To fix the problem:

  1. Insert your Windows installation disc and restart your computer. 
  2. Choose your language settings, and then click "Next."
  3. Click "repair your computer."


If you don't have this disc, contact your system administrator or computer
manufacturer for assistance. 

  File: \Boot\BCD  

  Status: 0xc0000001  

  Info: The Boot Configuration Data for your PC is missing or contains errors.

Any tips on how to fix this? I'm going to try to repair it with the installation disk, but I'm worried that some of my data won't be able to be recovered.

You've got a BIOS/CSM/legacy-mode Ubuntu installation alongside an EFI/UEFI-mode Windows installation. As you've discovered, this combination is not good. You probably got to this point because you followed misguided installation instructions that suggested you enable the CSM (aka "legacy boot support") in your firmware. This causes more problems than it solves, as described in greater detail on this page of mine. To fix the problem, I recommend:

1. Download the USB flash drive or CD-R version of my rEFInd boot manager.


2. Prepare a boot medium from the rEFInd image.


3. Enter your firmware setup utility. You typically do this by hitting Del, Enter, or a function key soon after powering on the computer (about when you'd press F12 to get the firmware's built-in boot manager that you're using as a workaround).


4. Disable the CSM. This will produce a boot straight to Windows, once you reboot.


5. If it's currently enabled, disable Secure Boot. You can re-enable it later, if you like.


6. Insert the rEFInd medium and reboot into it. (You may need to use your F12 key.)



7. rEFInd should show options to boot both Windows and Ubuntu. Test Windows first, then Ubuntu.


8. If you can boot both Windows and Ubuntu from rEFInd, then you can install an EFI-mode boot loader from Ubuntu. There are many options, but the two easiest are likely to be:
   
   
   
   
   • rEFInd -- You can install the rEFInd PPA or Debian package to use it, bypassing GRUB. You know it works at this point, so it's a pretty safe choice; but if you want to use it with Secure Boot enabled, there will be more hoops to jump through later, as described here.
   
   
   • GRUB -- You can install the grub-efi package, which should replace the grub-pc package. You'll then have to re-install GRUB by typing sudo grub-install. You may also need to type sudo update-grub to get the configuration file right. This is a bit of a leap of faith, but if it doesn't work, you can use the rEFInd medium to boot up and try again. Alternatively, Boot Repair can usually get things working by re-installing GRUB automatically. One advantage of this approach is that you should be able to re-enable Secure Boot after you're done with no additional trouble.
   
   

There are numerous variants on this procedure. Most notably, if you're sure you want to stick with GRUB, you can skip using rEFInd and boot your Ubuntu installation medium in EFI mode and run Boot Repair from it, rather than use rEFInd to boot into your existing installation and repair it.