Try plugging the Arduino in a different USB Root Hub on your computer.
By that I mean, plug it into a different USB port. Sometimes a Root Hub will be pre-split into 2 or more ports. So if you have it plugged into one of the ports in the front of your computer, try a port in the back (or vice versa).
Also, you could try turning off UAC (User Account Control), though that is Vista's new security system so I would not recommend it. But in my honest opinion it is more of a hassle than a blessing. :-/
Hope that helps!