|Can you clarify your setup please...|
Is the the third hub wifi actually enabled?
Is the printer's wifi enabled; and does it have a cat-5/ethernet option - and are you using that cat-5 option?
XP-pc is connected to the third hub that is wifi enabled?
Does XP-pc connect to third hub by wifi; or is connected via ethernet/cat-5 cable only?
Win-7-pc is on the same hub (or not); and how is it connecting to whichever hub it is on - wifi or ethernet/cat-5 cable?
Can you confirm the ip address and subnet details for the printer's wifi; and for both your pcs? Printer setup usually allows a printout of the settings for wifi and cat-5 (if exists on the printer). Also if the printer allows both wifi and cat-5 connections then likely wifi will not work well - if at all - if cat-5 is set to be used. I have a Canon that allows both, and on occasion it will work with both; but more often than not it won't allow wifi operation if cat-5 is active.
On your XP-pc and also the win-7-pc run the command < ipconfig /all > (without the < > ) from a command prompt (start\run and type: cmd - and press enter/return to bring up the dos box) - and copy the details here.
And have you installed/added the printer correctly within XP?
If you can determine the ip address for the printer (wifi) can you ping it from both computers?