There is a known issue from Microsoft about the Vista x64 printer sharing option. Microsoft provide a workaround for the printer sharing problem and i will share with all of you. I already tested and works fine with Vista x64 Enterprise RTM and Windows XP SP2.
1) Go to the XP Machine and go to Start - Control Panel - Printers and other hardware and then printer and faxes.
2) Double Click Add Printer to start the Add printer wizard and click Next.
3) Click on Local Printer, clear the Automatically detect and install check box, and then click NEXT.
4)Click CREATE A NEW PORT and then click LOCAL PORT in the PORT TYPE section.
5)In the Port Name box, type the path to the printer in the following format where SERVER is the name of the print server and PRINTER is the name of the printer:
6) Click NEXT and then select a Windows XP driver for your printer.
7) Click Next and the follow the intructions to finish the wizard.
NOTE: When you use this procedure, the print job is printed locally and then redirected to the network path. If you use this procedure, you do not receive printe updates from the printe server when you update the printer driver on the print server.
I hope this process help with your problem. It works for me and was so easy to do.