Recycle Bin Deleting More Than It Contains

I went to empty my recycle bin this morning. It contains four small files. But when I click "empty recycle bin" the pop-up says it's deleting thousands of files in the gigabytes range. Does anyone know what causes this? I believe it's happened to me before, probably on Vista, but I don't remember how I fixed it or if it just eventually fixed itself. I don't notice anything missing and the computer doesn't behave oddly, but I cancel the deletion before it gets too far.

I'm currently running a virus scan but it hasn't picked up anything yet and it's 75% of the way through.