Edit: Seems to be sorted now, typically I only thought up a solution after posting.
As of yesterday, Avast has been complaining pretty regularly that C:\jau38uj.bin\50BE4DF0031.exe is attempting to connect to a dodgy url (I'll avoid posting it for obvious safety reasons!). A bit of google-fu shows that this behaviour is linked to the Spyeye rootkit, which probably sneaked in via a bad google result I came across yesterday.
The problem is that, whilst Avast is seemingly preventing it from actually doing anything, it's also not actually detecting it. I ran a full scan last night and nothing came up, so I installed Ad-Aware and gave that a run, but again got nothing detected. The folder it's in is obviously sneakily hidden, and whilst the command prompt admits it exists when I try and use "del", it fails silently.
Thoughts on getting rid of it? If nothing else, it's annoying having Avast pop up every 5 minutes.