Gareth, let me pose the question;
Why are you trying so desperately to install GRUB from inside of Windows? Are you afraid of not being able to boot Windows after your F8 install?
The reason I ask is because if you are installing directly to a second drive there is nothing to worry about.
What would (possibly) temporarily
break your install would be GRUB getting installed directly to your MBR (Master Boot Record) on the first drive.
The simple solution to this would be to check advanced Boot Loader configuration during install and install GRUB not to the MBR, but to your /boot partition and then point the Windows NTBootloader to grub.
If you choose to do this I have a few links for you. (This is what I did.)
This post was in reference to someone else's problem, but with the amount of context I included it should easily explain the general problem.
As I note in that post I got EVERYTHING installed, and Windows' NTLoader still allowed me to boot into Windows no problem, but GRUB did not finish booting.
Stoat was nice enough to figure out what was not going correctly, which is that you cannot us KNOPPIX to do the "dd" command needed. You MUST instead boot using the Fedora 8 install DVD and run the command as Stoat points out here;
After this the Fedora 8 install boots perfectly.
I apologize if this isn't something you wanted to explore right now, it only seemed logical given your comments though. In any case, good luck.
ps, if you only want the GRUB installed you could of course so the entire install then delete everything except the boot partition.