I got this too and have found it to be a simple issue of one of my other programs locking the .jar file. To test this what to do is simply quit out of the ant script (though it has probably already crashed out!) then try renaming the .jar file manually in DOS. "rename myJar.jar newTempName.jar" and it will give you an operating system error message. So just keep retrying your ant script as you close down other programs you have open until it works.