How to change from a GPT drive for new Win 10 install

Hi all
I've had my laptop for many years & never had any issues until I stupidly decided to cause some (not deliberately tho- just stupidity!).

I had upgraded to the Win 10 Pro from Win 7 Pro when the free rollout happened and all was fine. But the laptop had become unbearably slow despite only being used for one or two programs & being regularly 'cleaned' (using Glary Utilities) -caches, cookies, registry etc. SO I opted to reinstall Win 10 fully rather than just do a reset because I'd done that previously & it hadn't made much dent in things. I thought the reinstall had gone fine until I realised it had installed the Home version & not the Pro I'd previously paid for so as I really needed the functions of the Pro I figured the only option was to buy a Win 10 Pro disc & reinstall that way.

BUT when I went to do that, the install wouldn't progress stating that "Windows Cannot be Installed to This Disk. The Selected Disk is of the GPT Partition Style”. I even more stupidly then opted to format the previous Win 10 partition (oh, & then delete it for good measure) thinking the problem was down to that installation so now am stuck where I have no Windows and no means of easily installing the new version. So I did what I should have done first & read up a bit about the error only to find that it’s to do with things I don’t seem to have on my laptop I.e. the Legacy mode UEFI – I’ve worked through the BIOS but found nothing that would seemingly help. I did cancel the install but obviously not before damage had been done. So now I’ve a dead laptop - & yes, I accept – my own fault entirely . I’m really REALLY good at fixing things until they’re well & truly broken sometimes! The 2 screenshots show what I get on booting up after I tried to repair the damage.

Only other thing to mention is that the drive has 2 partitions - one for Windows & the other for all my programs (& no of course I've not backed them up - duhhh - horse & cart come to mind??!!!)

SO before I make things any worse than I have already, can anyone help me figure out the best way forward? It's been suggested that I might need to format the entire drive rather than just the one partition, but I need to make sure that's the case. Is there any way I could maybe use a live Linux CD to at least save some of the data first perhaps?