Okay, now I have followed up on your last 2 links.
The second-to-last ("Install a printer on a home network") is just about the HomeGroup and Windows 7. It says XP computers cannot join the HomeGroup. It does not address my problem of how to share the printer across operating systems.
The last article also does not address my issue: Installing the correct driver for a network printer where computer A is on Windows 7 and Computer B is on XP.
I have read lots of board discussions here and on the HP website where people succeeded in doing exactly what I am trying to do, but they succeeded only after trying many things and in each case the solution that finally worked for them I either do not understand or cannot access. For example, one talks of installing the 64-bit driver for XP or Vista on the 32-bit XP machine. But they don't say how. When I try to do that, the HP website will not let me.
Another talks of putting the HP universal print driver for the 1320 onto the W7 machine. But how? When I get to the step where the W7 machine reports it is unsuccessful installing the printer as a network printer because it can't find a driver, and then I go to "browse" to give it the driver location -- I don't know what file to give it. The folder for the HP universal driver has many, many files in it. HP doesn't tell you which one is "the" driver.