Indeed, am I asking the impossible for a Mac to print via a printer connected to a Win Xp PC. Do I need the 'sharing' button pressed?
No, it's not impossible, but you need to ensure that some things are in place and configured correctly.
Here's a useful guide:
Printer Sharing - Windows XP Printer Sharing With Mac OS X 10.5
More via google (share windows printer with apple mac).
SD