Just an update from the Hardrive Failure Journal. The PC has been working for a few days now. Fixing it wasn't as bad as I thought it would be, although it did contain much more dust than I anticipated. That wasn't particularly pleasant to clean up - great clods of it all in the front fan filters and the front of the case
Thankfully, the actual installing of the new drive is mostly easy, if a bit time consuming. Windows was easy too. ...Except I made the mistake of having all the drives plugged in instead of the one I wanted the OS to install on. Windows, for some reason, thought it would be a brilliant idea to put the Boot files onto the drive it first sees, which is the one I keep all my personal files and documents on. I didn't notice this until after Windows and the majority of my programs were already installed.
I made the further mistake of making the OS drive Active instead, and had to spend the next hour or so using Windows Repair and the Command Prompt to build the Boot Manager files onto the correct drive so Windows would boot up. It was...an experience, to say the least. I learned quite a lot that morning, and I would be lying if I wasn't proud of myself for fixing it
So yes, everything is fixed and working again. Woo!