Some time ago, I observed something strange happening to my laptop: all of my menu entries in my start menu (and "all programs" menu) vanished, the task bar icons changed into a "unknown file type" icon, and nothing but Firefox was working any more. My personal files didn't seem to be affected, except for the folder "App Data". It still had all of its folders, but they were left empty.
I could literally watch the menu items disappear. I could do nothing against it.
A virus? A bug? I have no clue.
Very nice: all of my emails were gone, too. Of course. Including all of my vacation plans, confirmation emails etc.
A very good friend of mine pointed to a free tool called "Shadow Explorer". I learned that Windows 7 creates "Shadow Copies" of nearly everything on the computer. This tool enables you to see these copies (up to about 2 or 3 weeks in the past) and recover the files.
I was able to recover most of my missing app data files, including mail etc..