|In Win7 go to CP / Devices and Printers / and select the printer / click Customize this printer.|
On the security tab, do you have Everyone listed as permitted to print?
With that, I have had XP users without log ins able to use my printer.
But I have a Win7 log in, and that's really the reverse of your question. It may be that Win7 wants to allow only authenticated users to offer its printers to others.