Try this before the ant command
set JAVA_HOME=JDK LOCATION HERE
JDK Location is the root of the JDK installed, such as on mine.
set JAVA_HOME=C:\Program Files\Java\jdk1.7.0_45
then type SET to check if it worked, and now try your ant command. I also changed my PATH to point to the JDK\bin folder before my JRE\bin folder, but I think Java check JAVA_HOME first.