I've never even heard of some crazy stuff like that LMAO!

Good ol' Microsoft.

Is there any chance at all, that someone used your computer when you were gone? Any chance?

[edit] I just goggled this and there is some information about this online. For the few people this has happened to, it seems to always involve corrupt files. Some were profile corruption, others were system file corruption.
Need to do some research, as the explanation and sometimes cure, are very lengthy.
Hopefully you had everything backed up?