So, for the last couple of nights, the computer has spontaneously shut off and restarted without any sort of warning. This time, when it started up, it seems to have lost a couple of my email folders. So it looks like the computer will be going in to the shop some time in the next few days to be fixed up yet again.
This is beyond a joke.
LATE EDIT: After a lot of buggerising about I think I've managed to recreate the most important of the missing email folders (Outlook Express seems to like filling up the computer's recycle bin with backups) but this doesn't mean the original problem's fixed... will just have to wait and see what the news is.