Where is ant_root and how is it set? Aren't the two statements about where flexTasks.jar is located redundant?
I've never use Ant with Flex or Flash Builder, but I think ant_root is the equivalent of the ANT_HOME environment variable (mine is set to C:\apache-ant-1.7.0)
Senior Content and Community Manager
Never used Ant outside of Eclipse, but if you're using Eclipse it's probably easier to use the Ant view. No need to setup environment variables or anything, just drag your build.xml file into the Ant view tab area. (You may need to modify the config to add jars like flexTasks.jar under an entry in External Tools Configuration, can't recall.)
Ant home will be found in path_to_Adobe Flash Builder/eclipse/plugins/org.apache.ant_your_version_number