Using MAC OS with NAV 4.0 : "Remote Desktop Connection for Mac" 
http://www.microsoft.com/mac/downloads.aspx?pid=download&location=/mac/download/misc/rdc_update_103.xml&secid=80&ssid=10&flgnosysreq=True

other option:

Virtual PC for Mac with Win XP
http://www.microsoft.com/mac/products/virtualpc/virtualpc.aspx?pid=virtualpc