Windows mysteriously freeing up 8 GB of hard drive space is marbles

Over the past few months, my free hard drive space had been steadily disappearing. It had gone from about 15 GB down to about 5.5 GB, as of yesterday. Of course, I download a few things here and there, but all the while I've suspected that that degree of leakage could only be attributed to one thing: the arbitrary whims of Windows Vista. Well, today it has appeared to heal itself.

Mysteriously I've regained 8 GB from God-knows-where. Now the task begins to look through my important files (tax records, treasure maps, etc) and see how many of them Windows decided to delete.