Happily, my self-inflicted partition disaster proved easy to get fixed.
I concluded that fixing the problem required using the kind of utility programs only PC repair shops have on hand to get in and eliminate that GRUB Linux boot-loader, so I hauled it down to Dok Klaus in Warrenton.
Klaus had it fixed the same day and only charged me for one hour of service.
As PC problems go, it was ultimately minor. Now I have my entire hard drive to play with.