If two people use the same machine, and the first starts X, the second person can't start X unless they have administrator rights.
Cygwin sets /tmp to have mode drwxrwxrwt which means ordinary users can't remove /tmp/XWin.log if it's owned by someone else. So subsequent attempts to start X fail because it can't write the log file. My suggestion would be for X to create the file called /tmp/XWin.log.$USER luke PS: Note that unlike Unix, it is possible for a user to remove a file that some other user created in a directory with the above permissions, provided they have administrator privileges on a machine.