If you could use Ant then you would just use the JUnitReport task as detailed here: http://ant.apache.org/manual/Tasks/junitreport.html, but you mentioned in your question that you're not supposed to use Ant. I believe that task merely transforms the XML report into HTML so it would be feasible to use any XSLT processor to generate a similar report.
Alternatively, you could switch to using TestNG ( http://testng.org/doc/index.html ) which is very similar to JUnit but has a default HTML report as well as several other cool features.