I once had a user call because her computer wouldn't boot. I ended up pulling the hard disk and putting it in another machine so I could recover some of her files. When I looked at it, I noticed a bunch of folders on the root of the disk with three letter names: DLL, EXE, INI, SYS, BAT, etc...

The really impressive part is that she had actually managed to move most of the system into these before hitting the files that were in use.