Install the printer on your Windows 7 machine, with the Windows 7 driver.
Share the driver, and get the path name for it (i.e. \MACHINENAMEPrinterName).
On the XP computer, do the following:
1. Start the Add New Printer wizard
2. Select a Local Printer, do not auto detect
3. Choose a New Port->Type is Local Port
4. In the dialog, type \SERVERNAMEPrinterName as it appeared above on the Windows 7 machine
5. On the driver selection page, either use Have Disk to get the driver, or select from the list
6. Continue with the wizard until finished
7. You can get the hostname (\MACHINENAME) either from System Properties, or by running HOSTNAME at the Command Prompt. The Share Name is what the printer is shared as in the Printer Properties->Sharing tab