Thanks for the answer!
if i run from the prompt of windowsXP:
java -jar sigar.jar ifconfig
i got the normal response:
MS TCP loopback interface
l0: Link encap:local loopback
....
RX packets: 11251 ...
TX packets: 11251 ...
collisions -1
RX bytes 3919679 (3.7M) TX bytes 3919679 (3.7M)
I got the numbers running the program one time, but i always get similar results: with the same RX and TX packets, and the same RX and TX Bytes!