try the small antenna 3" long approx that came w unit, try moving antenna to different spots; for example if near a window, try moving a few inches away from window or screen. Its kind of odd you mention vista as I have a newer hp laptop w vista that I cannot get the alfa to work w that part OS although w windows 7 rc, it works great as long as you initially use the cd to install the drivers then use windows update to search for updated drivers by opening device mgr, then update" allow windows update to update driver" option.
You will find(listen up everybody) that w the same hardware setup, antenna pointed the same direction w the most recent drivers, your download and browsing speed will be 10 to 20% faster with windows 7 rc as compared to windows xp!! (this done w a 500mw alfa adapter w stock small antenna)