Hi Radu,
Which version of NI-VISA are you using?
Please be shure to run the latest version. Right now this is NI-VISA 3.0.1. You can check your version information in Measurement and Automation Explorer in the Software section.
The following link should lead you to the download of the most recent version:
http://digital.ni.com/softlib.nsf/webcategories/85256410006C055586256BAC002C51FA?opendocument&node=132070_US
Regards,
Richard H.
National Instruments