plug device directly into usb on back of pc - instead thru the hub - c if it works - or recognises it?
Look at XP event log to c any usb related messages in event viwer
chk device manager and c that all usb devices on the tree (usb tree) are operating.
maybe usb cables are defective?
xp does not like more than 5 hubs connected in a row?
chk of usb device is drawing more than 500 miliamps more than bus can provide - use a sep power supply (self poweed) if it needs more power.