1. Do you have the java developing kit? This is different than the normal java runtime enviroment. Only the jdk has javac. If you don't have it, you can download it here: http://java.sun.com/javase/downloads/index.jsp
2. Make sure java_home is set to C:\Program Files\Java\jdk245235 (the numbers depend on ur version).
3. Make sure the nxt_home is set to: c:\mydirectory\lejos_nxj. There can not be any spaces in the path. If this still gives errors, try: c:\mydirectory\lejos_nxj\lejos_nxj.
Respond back if these help you or don't!