Pretty much overnight, the amount of free space in my local disk (C drive went from almost empty to almost full. I have a feeling I have a virus, though virus scan does not reveal anything out of the ordinary. Any suggestions?
can you see what the drive has been filled up with?
You could try using a different security app like AVG or MSE?
Also why not try a chkdsk too? Just right click on your HDD and choose properties, Tools, check now, remembering to tick the box for repairing broken sectors...