Sorry, bit of a problem in trying to provide a memory dump. Didn't have time to deal with it initially so I left the computer as it was, with avast uninstalled. Someone in the family using my PC briefly installed AVG while I was away, upon realising that I didn't have a virus scanner installed; when I removed it (and ran their remover tool to make sure it was completed removed) and put avast back on, the previously mentioned login problem was no longer occurring. And, for further annoyance, my earlier System Restore points aren't working and I've no idea why, so I can't set my computer to before AVG was installed.
I've tried everything short of formatting the PC and starting from scratch to try and reproduce it and came up with nothing. If you reckon it's very likely to appear again after starting from fresh I can try it, but I'm not entirely sure when I'll have enough time spare to sort out a full format and reinstall properly - and it could have been down to how I installed other programs, which I don't remember the details of. If not, I'll keep an eye out for it happening again (especially if I have to format the PC at any point) and make sure I get a memory dump before any outside interference can get involved next time.
Either way, apologies for wasting anyone's time.
