You need to set your JAVA_HOME environment variable.
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.
C:\Program Files\Java\jdk1.7.0_45
set JAVA_HOME=C:\Program Files\Java\jdk1.7.0_45
then type SET to check if it...