I'm assuming it's a USB printer connected to one of the PC's with a wireless card, yeah?
if you have the windows XP firewall on that PC then the other PC's won't be able to use the printer. that could be a reason. I had this problem, until i removed the XP firewall and added Zone Alarm firewall and placed the other PC's private IP address into the "trusted" zone which allows printer and file sharing across the firewall.
It's a long shot, but this could also be your problem