No-one has answered you on this one.
Can you confirm that its not when plugged into a different USB port.
I went through this with an UNO that wouldn't work on Win2K, and hence worked out what was going on in both 2k and XP.
What I found windows does is here http://arduino.cc/forum/index.php/topic,60318.msg436416.html#msg436416
In theory the machine finding new hardware should recognise it, and use the same driver that's already installed.
I believe there may be some later boards that have a different VID/PID number, which will look for a different driver.