Specifying directory of 'jarsigner' command for 'signjar' task

Hello,

today I had the problem, that the 'signjar' task threw an error message like
  Execute failed: java.io.IOException: CreateProcess: jarsigner.exe -verbose -keystore mykeystore myjar myalias error=2

I learned, that the working directory of jarsigner.exe had to be configured in the PATH environment variable (of Windows XP Prof.); afterwards the task run successfully.

Anyway:
Is there an option or a way to configure the working directory of the jarsigner command within Eclipse to avoid putting the working directory into the system PATH of the OS; if yes, how?

I work with Ant 1.7.0 and use it as a plugin within Eclipse 3.4.2.

Thomas Wiedmann