I thought I'd wait a while before posting back in case of any issues, but all has been well. Thanks to all those who suggested options above, and here is what I did in the end:
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Bought a 500 GB USB external drive to back up the existing data on the C drive just in case
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Bought Win7 64bit on a bootable USB stick for under £20 (there are lots of computer companies selling these from what they claim are un-servicable computers which were due for an upgrade but they wrote them off instead, leaving them with 'new' unregistered software. They offer to ship the damaged PC to you as well to sort of comply with the spirit of MS terms, but there is a fee. They can instead dispose of them via WEEE regs FOC. Make of all this what you will.)
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Added win7 64bit as a dual boot on my SSD. To do this I needed to partition my SSD but windows built-in software would not do it. Reading around suggests there may be a file stuck in the middle of the drive which mirrors data across. Anyway, downloaded Minitool partition wizard (which was great) and it created the partition without a problem. I went with around 50GB.
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The win7 64bit install went fine, and the authorisation code worked. But after using it for a while I noticed messages about not having a registered version of Win7 and wondered if it was the start of trouble. I only had 30 days to sort it out. It wanted me to use the automatic MS phone system to register the OS. In the end I did this and the messages stopped plus in the control panel the status changed to 'activated'. It has been fine since. I vagely remember having to do this for win7 32bit but it was at least a year or so ago.
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The only remaining annoying thing was that when the computer was switched on showing which dual boot system to select it listed both as Window7. I think 64bit was first but there was no way to tell. More reading around and I found 'Easy BCD' which turned out to be another great bit of software which allowed you rename the OS, change the order, plus alter how long the count down timer runs for before selecting the 1st option automatically. I changed one to 32bit and the other to 64bit, put the 32bit one first, and set the time to 4 seconds. This way I can switch on, do something else for a minute and 32bit OS will be ready when I come back. Also the other family members use this PC so it is easier for them. If I need 64bit I just need to select it quickly on the dual boot screen at the beginning. Actually it's quite nice to have 64bit fire up and be just for me! It just has Fusion, the CAD software and that's it, none of the other bits and pieces and other non CNC related software which seem to creep on PCs over time which makes it very quick to start (although win7 is pretty quick to get going anyway).
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Last thing to do (!) was download Fusion 360 and have a play. I've done a bit of that now and it looks great. Created a few toolpaths, some gcode, some simple CAD, and it all looks straightforward enough although it is powerful and its going to take some getting used to all the options. Not cut anything with it yet though.
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Best news of all is that I do not have to re-boot between CAD and CAM - I've managed to installed my CAD programme as well and so far it has worked without a problem on 64bit which is great! Now I can do the CAD, create a step file, open it in Fusion and create toolpaths. I'm not sure I'll ever do serious CAD work in Fusion as all the files are stored on the cloud - I know you can have a local copy for a number of days but I'm still not sure. A big design can take days, weeks, or months to develop and I'm not ready to store that outside of my machine just yet.
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Anyway in summary very happy with this setup for anyone considering the same sort of thing.