Why does a .DS_Store file keep appearing on the desktop?
Type this in Terminal (/Applications/Utilities/Terminal.app):

defaults write com.apple.finder AppleShowAllFiles FALSE
killall Finder


The first command sets the hidden files visibility to hidden; the second one forces Finder to restart. This is needed for the first command to take effect. 

You can also log out/reboot instead of killing the Finder, but the first command is needed to change a hidden preference (one that the Finder can’t change using the normal Finder view options or preferences).