Posting this in chillout as I actually already fixed the problem, I'm just curious to see whether anyone else had this issue.

I installed the ISO version of the 8.1 preview in a new partition and 8.1 replaced the 8 Boot manager with it's own one (or at least I got a different splash screen).

Anyway long story short, I ended up not using 8.1 because my USB 3.0 drivers wouldn't work with it, so I went back to 8 but left 8.1 on there (along with it's boot sequence). About 2 weeks later, suddenly my PC started blue-screening, and when I finally coaxed it into booting into 8 (by messing around with advanced options and forcing it to boot 8 rather than 8.1). I got a message saying that there were "drive errors".

Checked the HDD, nothing wrong with it, and once I got 8.1 off of there, everything has gone back to normal.

You tell me though, how on earth does an OS corrupt itself without even being used!?