This time clean install seems to have worked, so I can now install software the way I used to. I also deleted all unnecessary partitions on disk 1.

I was hoping to at least find the cause as I...

Yes, exactly.

Okay, well since I already did a clean reinstall I might as well try again. Will let you know how it turns out.

Thanks for the help.

As mentioned I installed booted from a Windows disc.

I didn't erase the partition, I just formatted it. Created Program Files folders and installed to that. Which is what I find weird, because...

Ah I get where this is going.

It's a long story which basically boils down to not having anywhere to go with things and laziness... This PC has been through some tough times. xD

Do you think I...

I think I did more or less as in step 13. See disk management screenshot.
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So recently my Windows 10 install crashed so I had to do a clean install on my SSD. (C: Drive)

Normally I install my programs on the HDD (D: Drive). I cleaned out all the old programs along with...