People deleting Windows and/or formatting partitions, or even worse
formatting entire hard disks thereby deleting all data don't do so because they know what they are doing (because they KNOW what the problem is) but because they have no idea what they are doing because they have NO IDEA what the problem is. It is the 'Quick Fix' option used by people who have far too little knowledge about computers. It is an action not based on knowledge but based on ignorance. Which is fine if you are an average consumer of course but it is toe-clenchingly bad when you claim to be a professional.
If these people have formatted your entire disk thereby deleting your photos that is simply inexcusable. And you only use a clean room when you need to open either a seriously defective hard disk that you cannot access normally anymore or you need to use specialised equipment to retrieve valuable data that has been erased and even overwritten because with the right equipment you can retrieve data that has been overwritten. But then we are talking about seriously valuable data or forensic investigations here. From what you describe I have no reason to suspect a serious, or even any problem with your hard disk. And all you need to do to access that drive is to connect it as an extra (second) drive to another computer. No duplicate drives or any of that. You could even simply put it in en external USB enclosure and attach it to any computer and all data still on it should be accessible that way.
The question is whether these people have indeed formatted your entire drive or not. That will determine whether it will still be possible to access your photos. It can depend on how it was partitioned. Did you have only a single C: drive or also other partitions indicated by letters D,E, etc.? Any optical drive(s) (CD or DVD) will also have a drive letter assigned. If you only had a C: drive indicating your hard disk and a D: drive indicating your optical drive and nothing more then there is a serious risk these people may have deleted your data when it was completely unnecessary and they could have (and should have) copied it to any other storage medium with very little effort.
Do I now understand correctly that you have used the computer again after they had reinstalled Windows? Were you able to ascertain that the photo's were no longer on the hard disk? Do you know in which folder they were located and that this folder wasn't there any more? If that is the case then there is still a chance your photos can be recovered but it will be more troublesome. But there are utilities that can scan an entire drive for any files that can still be recovered. It will depend on the formatting method that was used if indeed it was formatted. If they used a Quick Format there is still a chance but if they used a Full Format then it becomes MUCH more difficult. Then you are getting into the realms of clean rooms and fancy equipment as described. You should ask them what EXACTLY they have done with your hard disk. Ask them to describe it in an e-mail they can send you. And ask them if they formatted it. And if so what utility they used and with which settings/parameters and/or how long the formatting process took.
From what you describe the PC still does it is clear this is not a 'dead' PC. It could be simply badly seated RAM modules or a defective RAM module, which can be tested and fixed/replaced. Or a cooling problem. Or a dusty or indeed close to failing PSU (Power Supply Unit). All kind of things can go wrong when a PSU starts to go bad. This can even happen when for instance you upgrade to a faster graphics card that needs more power and you PSU is not powerful enough to supply that increased demand. You didn't make such a change/upgrade recently by any chance? Or even a while back? An overtaxed PSU can in time become unstable. Or it can be just a badly written driver or..... Lots of things. But a lot can be determined if you know where to look and take the trouble to look! But "So it is the CPU".....? Oh boy!
Enough for now I think.
When in doubt..... Press the shutter.