Multiple arduinos, resetting and USB.

Hi, Since a few days I've got 2 Nanos beside the duemilanove I've been working with until now. All are recognized by PC and work perfectly by themselves.

When I upload a sketch to one of 'm or open the monitor-window, all... arduinos instead of just one reset for some reason though.

Does anyone know what's causing this and perhaps know how I could configure my PC (windows XP) to keep the other ones running while I'm uploading a sketch / monitoring a specific one ?

Are they connected to mainboard USB ports? Thru a hub?? What OS?? Hmmm...