I had been puzzling for months over the amount of free space on my ThinkPad laptop - it claimed to have only 5 GB free out of 50, but adding together all the visible files barely got me to 25 GB. None of the various defragmenting and data scrobbling tools made much difference.

But I found the answer. The Thinkpad's automatic backup system had set up an extensive and unnecessary catalogue of system restore points. Deleted them all, and, hey presto, 20 GB liberated just like that!!!!