code: public static boolean isInstalled(int major, int minor) {
          try {
              // see http://support.microsoft.com/?scid=kb;en-us;315291 for the basic algorithm
              // observation in my registry shows that the actual key name can be things like "v2.0 SP1"
              // or "v2.0.50727", so the regexp is written to accommodate this.
              RegistryKey key = RegistryKey.LOCAL_MACHINE.openReadonly("SOFTWARE\\Microsoft\\.NETFramework");
              try {
                  for( String keyName : key.getSubKeys() ) {
                      if (matches(keyName, major, minor))
                          return true;
                  }
                  return false;
              } finally {
                  key.dispose();
              }
          } catch (JnaException e) {
              if(e.getErrorCode()==2) // thrown when openReadonly fails because the key doesn't exist.
                  return false;
              throw e;
          }
      }
docs: Returns true if the .NET framework of the given version (or greater) is installed.