Hi Roberto,

As you are using Foobar you are part way there already. It needs some help to avoid windows software which automatically re-samples / up-samples audio. For XP this is ASIO4ALL, for 7 it's WASAPI, There's loads to be found on this if you google it.

There's still potential for jitter so try different USB ports too, it they are spread around your notebook they may sound different due to their position and the wiring in between them. Some of the USB to SPDIF converters do a good job too, eg M2 Tech Hi-Face.