Passing an SMB username to CUPS?

[MacMasta]

I'm busily setting up cups, and have run into a snag.
The printer I'm trying to get to is on my parent's windows XP machine (sorry, I'll get them converted someday). However, CUPS doesn't seem to have a setting to pass an SMB username to that machine, so it just drops the incoming print request.

Process: I set up the printer according to the built-in web-based interface - but jobs hang in the local queue.

I figure I need to hand it a username because when i use LinNeighborhood to browse the SMB network, I cannot see any of the shares for the XP machine until I do "scan as user" and enter a username that's valid on that machine.

Where do I tell cups about this?

~Mac~