It sounds like you have it setup right. What happens if you try something like this, which worked for me:
C:\>set ANT_HOME=C:\apache-ant-1.8.1
C:\>set JAVA_HOME=C:\jdk1.6.0_24
C:\>set PATH=%ANT_HOME%\bin;%JAVA_HOME%\bin
C:\>ant -version
Apache Ant version 1.8.1 compiled on April 30 2010
This also worked for me by setting up environment variables, like so: