At first, i was worried I had a virus or something, but then I remembered about Vista's Shadow Copy feature, which copies old versions of files so that if you make a change and you want to undo it, you can restore a previous version.
I found this article which explains how to dig into the settings for Volume Shadow Copy. It turns out that somehow, Vista decided that the shadow copy settings for my D: should be UNBOUNDED, which means, copy until the drive is full!
I was able to use VSSADMIN to fix it, following the directions of the posting. I simplay ran this command as an administrator:
vssadmin Resize ShadowStorage /On=D: /For=D: /MaxSize=50GB
Note to Microsoft: Volume Shadow Copy should default to a safe value, and not to UNBOUNDED!