This is difficult to diagnose as there is not enough information:
What is being built and how is it being built?
What is the destination of the jar?
Are the classes being compiled?
Is this better addressed in an Ant forum or is this directly related to a JBoss specific problem?

Time-poor developers helping out here are probably going to answer questions that don't involve too much heavy diagnosis work and the facts are nicely laid out.

For example, you might not be able to copy the jar to a JBoss lib directory with JBoss running in Win2K because Win2K locks the file. But we don't have enough information at the moment to help in any way.

check if u have sufficient privileges to create a file in your jar destination directory.