Only tool on first screen is memory diagnostic, which unsurprisingly finds no problems there. Exiting that test takes me into the DVD's bootup, which offers the Repair Your Computer option that I've tried repeatedly and doesn't work for me.
Evrry boot option from the extended menu offered at F5 | F8 takes me into either the DVD's automatic startup repair routine, which I've tried repeatedly and doesn't work for me, or into the DVD's bootup as above.
Robin: TVM for that link. Messing around with the various tools confirms my initial reckoning, that I'm savvy enough to know what the problem is but not solve it! I still can't get further than an initial Win7 screen (which I mis-identified above as being from the DVD) that puts me straight into Startup Repair but can't repair the startup.
Zeppelyn: doesn't get me any further on, I'm afraid.
Miké: had tried both of those before I first posted.