JUNIT testing working but strangly or biasing with dos window

[manglesh jain]

when i am running junit test this is showing error..

file:../common_build_tasks.xml:410: Could not create task or type of type: junit
.

Ant could not find the task or a class this task relies upon.

This is common and has a number of causes; the usual
solutions are to read the manual pages then download and
install needed JAR files, or fix the build file:
- You have misspelt 'junit'.
Fix: check your spelling.
- The task needs an external JAR file to execute
and this is not found at the right place in the classpath.
Fix: check the documentation for dependencies.
Fix: declare the task.
- The task is an Ant optional task and optional.jar is absent
Fix: look for optional.jar in ANT_HOME/lib, download if needed
- The task was not built into optional.jar as dependent
libraries were not found at build time.
Fix: look in the JAR to verify, then rebuild with the needed
libraries, or download a release version from apache.org
- The build file was written for a later version of Ant
Fix: upgrade to at least the latest release version of Ant
- The task is not an Ant core or optional task
and needs to be declared using <taskdef>.

Remember that for JAR files to be visible to Ant tasks implemented
in ANT_HOME/lib, the files must be in the same directory or on the
classpath

Please neither file bug reports on this problem, nor email the
Ant mailing lists, until all of these causes have been explored,
as this is not an Ant bug.

Total time: 2 seconds

But if i close this window and open the new window and run again then it is running iutest.....biasing with window....what is going wrong with this....
please help me out....can you help me why it is showing this strange behaviour......