Here's a mysterious one:
I just ran the Security Tango (look it up if you want), and after that I noticed a folder in c:\ that I had never seen before. It is called MSO Cache, and it contains, apparently, Microsoft Office.
I can't imagine that a series of virus scans would uncover a hidden Microsoft Office in my Windows 7 install but maybe it did. Or maybe I'm being spoofed somehow. Or maybe I'm crazy.
Can anyone explain what happened?