It's not intuitive, but try this:
ON THE XP MACHINES:
1. Open Printer and Faxes
2. Click File, Server Properties.
3. Click the Drivers TAB.
4. Find your shared printer in the list and remove the driver. Click OK all the way out.
5. Remove the shared printer in the list.
ON THE VISTA SERVER:
1. Open the Printers and Faxes
2. File, Server Properties, Drivers tab
3. Check to see an XP driver exists for the printer you're trying to share.
4. If yes, proceed to next section. Otherwise download XP drivers for the printer, then Add them in using the "add... " button on the "Printer Server Properties" page you were just at.
Here's why: when the XP machines try to connect to the server, they will try to download the driver from the server. If they can't find the driver, they will either prompt for one or use a generic one. It's always best to have the clients use the same driver as the server.
REINSTALLING THE PRINTER - ON THE XP MACHINES:
1. Find out the hostname of the server.
2. Click Start, Run...
3. Type "\\servername" and press enter
4. You should see the server come up with any shares listed, but most importantly PRINTERS that are shared.
5. Simply double click on the printer name. This should force the driver to install.
6. Try the test again.
XP is going on 10 years old now so it has quirks, but at least this means the fixes are mostly documented

Kinda like our trucks... .