Okay so I just got onto my parents computer because they complained about it being slow. I checked how full the HDD is, it says they've used a good 650gb. Well, considering the only thing they do on there is web browsing, excel files and some photos, this didn't make any sense. So I downloaded a file scanner to see what was taking up all the data.
Here's what it found
Under program data, in the folder iecache, there is a folder called nspscr. It alone is taking up 433.4gb!
Upon looking that folder up, it looks like it could be a program called "Net Spy Pro" as it creates folders with the above mentioned name within the program data folder.
Any insight, whether confirming that is what it is, or how best to solve the issue, etc, would be appreciated.