As an additional observation to the above advice, I can offer the following tips when it comes to getting a genuine Win 7 iso. First of all, the direct link to the Microsoft website above wouldn't let me download the iso, even though I have a valid product key, extracted from my PC, as it's an OEM install. I rang Microsoft to confirm this, and they told me that the only way for me to get hold of the Win 7 iso for my machine was from Lenovo, as it was an OEM version installed at manufacture.
The link from le Pingouin wouldn't work at first, as I removed Internet Explorer years ago, and it only works if you have internet Explorer 11, apparently. After I downloaded and installed Internet Explorer 11 the utility programme seems to work OK and I've now downloaded the last version of Win 7, with SP1, from the Microsoft website using it.
I've also downloaded the latest convenience roll up, and worked out how to remove all the added telemetry updates from it. The next task will be to create a slipstreamed iso with Win 7 SP1, plus all the latest updates, minus the telemetry ones, that I will probably put on a bootable USB.
Then I'll swap out the hard drive for the new spare I have and do a clean install and see how it goes.
For Andrew's benefit (if he hasn't already done this), I will try and see what sort of speed differences there are between the old and new installations. My gut feeling is that a new, clean, install should run faster. One reason for thinking that is that I've noticed, even after running CC Cleaner, that there are still a lot of legacy software installation bits and bobs around, with loads of empty directories and a fair few registry entries that don't seem to relate to anything now, left over from when I've installed software and then removed it.
The worst seems to be from Office 2007. I bought a copy of Office 2007 some years ago, to replace Office 2003. However. I simply couldn't use it; the "ribbon" drove me nuts, and it was taking me far, far to long to find out how to do really simple stuff that I was very familiar with, so I uninstalled Office 2007 and reinstalled Office 2003. However, it seems that Office 2007 failed to properly uninstall, and left stuff all over the place, including a load of what look like recent Microsoft updates that only apply to Office 2007.
Hopefully when I've finished, I should get the machine back to something like it was when I first bought it, several years ago. That will suit me jsut fine, as I rarely use it; it's been dual boot for years, and runs Linux Mint 99% of the time. It only gets booted into Win 7 when I want to run Autocad, and AutoCad was far too expensive for me to just dispose of and learn something new that would run on Linux.
Thanks for the advice, and my apologies for butting in on Andrew's thread.