Guys, thank you all very much for your advice and help.
The problem seems to be solved and I think I know now where I went wrong.
If you look at my #18 post it shows the UEFI settings AFTER the second test install on my SSD drive.
But after I inserted the MiniTools DVD, booted and hit F8 (which offered a direct choice of boot source), I found an arrangement I'd never seen before. The listing said this: (top to bottom)
SATA6G_6: HL-DT-ST DVDRAM GH24N
SATA6G_5: ST3120813AS (114473MB)
UEFI: HL-DT-ST DVDRAM GH24NSD1
Because there were two listings for the DVD drive (1st and 4th) it occurred to me that because the 4th was shown as 'UEFI', the 1st might be MBR, and was probably the option I should use for my next install of W7.
(Rootman has just mentioned this in post #21. Thanks again for another good call, man)
So I unmounted the SSD and mounted the old 120GB HDD. This disk was carrying my first W7 test install, also in GPT.
Using MiniTools, I deleted the 3 GPT partitions and wiped the disk. Then chose 'Create Partition' and selected MBR.
Then rebooted with the W7 DVD disk and the installation went to an MBR partition. Problem solved.
In retrospect, I think if I'd been more familar with UEFI I might have spotted the 'two-DVD' choice on my first test install, but I must have selected the UEFI option without being aware of its significance. However, I think the guys who write UEFIs and BIOSs could do a lot better with their explanations and terminology, even though it's mostly techs who browse those configurations. I mean, everything else is being dumbed down, so why not computing? It took me a while to get a handle on BIOS, back in '95 when I first started building PCs, but UEFI is several more levels of GeekSpeek above BIOS, probably because of the greater choice of options that have been crammed into the configuration. Never mind the quality, feel the width.
LOL, not as sharp these days, 'forty' represents 1940 :-).
Again, my thanks to all who've responded.