Installing language pack on a multi-boot system fails

I have Windows Ultimate x64. It's a fresh (re)install. After retrieving all available updates (including all optional drivers) and rebooting, I tried to install the Japanese Language pack via Windows Update and after maybe around twenty minutes of the software trying to install, Windows Update reports an install failure. The error message is:

Code:

Code 80070643: Windows Update encountered an unknown error.

I Googled the error code, and Microsoft says that the error is caused by a possibly corrupt .NET install. After using their .NET cleaner and reinstalling all the applications, the error persists. I looked further on Google and read a post by someone with the exact same problem as me. It looked like he was able to fix his problem pretty easily, and he explains how he resolved it in his following post, but his explanation is a little over my head. From what I can tell he used bcdboot to tell Windows where the reserved system partition is or something like that. But without explicit instructions I'm kind of in the dark. I'm not good with Windows. I prefer Linux. Basically I need what he did explained in Captain Dummy talk... I think his solution is the solution to my problem as well.

Hey neat! I fixed it myself by guessing at the guy's instructions!

What he did was issue the following command:

Code:

bcdboot C:\windows /s C:

And marked my system partition as active by using the administrative tools. I thought this would remove my boot loader, but it didn't, I still have GRUB. I think I basically just gave myself write permissions to the system drive, which Windows needed to install the language pack.