I was reading this very interesting article on a guy who noticed that when going from Vista SP1 to SP2 windows would almost finish the process (taking quite a while, apparently) and then it would report an unknown error and rollback all the things that it had done (wasting CPU and real time, by the way). After seeing the problem show up a couple of times the person realized that grub was there in the MBR. Replaced the MBR with Windows', tried to update it it was done. Great.
So.... a very interesting read, I have to say. Then I hit the comments and what do we find? None other than people saying that it's grub's fault. Say what?
What's there for Windows to see that belongs to grub? Not much, really. The first phase bootloader, located in the MBR, in other words it's comprised within the first 512 bytes of the HD. The second phase bootloader (which is called from the first phase bootloader) is located somewhere within the GNU/Linux partitions set up at the box. So, the only thing from GRUB that Windows can actually see (unless Windows is capable of reading out of the box ext2, ext3, ext4 and the other gazillion FS that we have available in GNU/Linux) is the first phase bootloader.
In my opinion, it's something as simple as old Microsoft's motto in action: "It's the Microsoft way or the highway". The update process is taking a look at the MBR and notices that's it's not Windows' bootloader. "Who in their right mind would dare install something on the MBR that's not made by Microsoft?" I bet they think there at Redmond. End of the game, let's stop the update process... _and_ (specially) not tell the user what's going on. It wouldn't be as insulting if at least they would suggest the user to replace the MBR with Microsoft's tools. You know, it can be replaced with GRUB a couple of minutes later after shutting Windows down after the upgrade process is done... but what do we expect from a OS that was made to resemble black magic, anyway?
As I have already said before:
Windows equals esotericism
GNU/Linux equals determinism