I've spent two days on this and have made no progress.
The problem machine here is running XP SP3.
The mbox 3 works fine on two other computers. One is running XP home SP3 and the other is running Windows 7 professiona 64 bitl.
A mbox 3 pro works fine on the problem machine.
I have completely uninstalled all drivers & pro tools. I checked the registry to make sure that everything was gone. There were some drivers hiding in the prefetch folder but I got rid of them too. I even uninstalled and reinstalled the USB drivers. No joy.....the light show just keeps going on.
Another thread here suggests getting a card for the USB ports.
http://duc.avid.com/showthread.php?t=308827
I will try that and report back in this thread. That is going to take a few days but I will keep hacking at this. I just don't get it.....this machine used to work ok with the mbox 3. Maybe a Microsoft update has done this.....I don't know.