Problem Setting Up Network Printer via XP

I am a new, just created, VISTA user.
I have it installed on a test machine which is networked to a second
machine which is XP PRO SP2.
The networking is via a D-Link router.
I have two printers, one an EPSON R320, the other AN HP PSC950 AIO.
The EPSON is connected directly to the VISTA machine via USB.
The HP is connected directly to the XP machine via USB.
Each printer prints just fine from the machine to which it is
connected directly (USB).
I have networking set up on both machines in common WORKGROUP
'mshome'.
File sharing works just fine in BOTH directions.
I can print from the XP machine to the EPSON via the VISTA machine via
the network.
BUT I cannot print from the VISTA machine to the HP via the XP machine
via the network despite the fact that the test print seems to got
through the spooler's without a problem (the document disappears from
the queue as if normal.

Seems weird!

Can anyone suggest where I can look for wherein the problem lies?

Thanks

-GECKO