...I finally did manage to get the dual boot working, and it turns out it was largely me (and not Redmond) who managed to bollux up things with Windows in the first place.
Short story is: I do now have dual boot working, although I'm using two harddisks to get it done, and not just the one. Not perfect. Not quite the way I had intended to set things up. But I don't need perfection really.
It turns out at least some of the issues I was having with the Windows installer (specifically it's sudden frightening insistence on formatting my (working) NTFS partition before it would re-install itself) were probably caused by me in the first place.
<Hangs head. Kicks at imaginary pebbles. Shuffles feet....>
I had removed SP4, and in a further desperate attempt to isolate some performance issues, I had renamed some Windows files temporarily in order to disable them...
...and then managed to get myself distracted to the point of forgetfullness while trying to install Windows and then Mint to the new disk. Oops.
No wonder then that the installer decided the disk was corrupted.
As it stands now, I've managed to reinstate everything, at which point the installer agreed it could re-install Win2KP-SP3 WITHOUT needing to format. So things are working once again on the original Windows disk about as well as they ever worked.
Pretty much only gaming/simming though. Nothing more webwise on the Windows side for me--I've moved all of that over into Mint and I'm quite happy having done so.
[Edit--specifics: I disabled the big, 320GB, new Linux C: drive in BIOS, which left the Windows (installer) seeing only the one, original 120GB, "D:" disk (NTFS format). I then attempted a "repair," but it didn't solve the issues, so I finally executed a full reinstall of Win2KP-SP3. Then AV install, followed by full systemscan and tight firewall install after that. Only then did I finally--physically--re-enable my web connection on the Windows side.
It works. I only use it for gaming/simming though now.
Windows has no idea the bigger (Linux) disk even exists on my system. I download and transfer files TO Windows from the Linux side using the mintDisk automount, but I can't go the other way right now. Suits me just fine.
BTW (and this is for Scorp 123)...since I did finally "punt..."
...I have a "default" install of Mint on the big disk--so my GRUB lies is in the root partition (of that disk).
Windows is happy too in it's own root--it just doesn't realize it actually lives on "D:" instead of "C:."