The Windows installer seems to place the installer background image (the yellow/golden colored epic scene where two opposing armies confront each other) in C:\ for some reason. I'm not sure why it can write there anyways, but it seems to be able and do so. The fix would be not to place this file anywhere other than, if needed, a temporary directory.
I may be mistaken in that this file was placed there by an earlier version of the installer/game (so not 3.6.0.3), but then you'd be aware you already fixed this...
Edit by Omega: Changed title to uninstaller