Everyone likes a happy ending, right?
I tried the reboot thing, and still I got the ...51 IP address for the Win7 box. After some fooling around, I realized I needed to check the Win7 TCP stack, and sure enough, at some point I had specced a static IP for it. That never was a problem before, and shouldn't have been now, but more recently I had changed the DHCP range in the router, inadvertently to overlap the Win7 fixed address. So the router was assigning one address, and the Win7 box insisted, rightly, on using its own. Mea culpa.
So I removed the Win7 fixed IP and rebooted it, and suddenly it's Springtime. Not only can I see Win7, but I am correctly queried for the access password to enter in.
From the Win7 side, I can see the PCL box and enter in.
Awesome, take a breather, and take a bow. Thank you for your help, especially you, muungwana. You are both knowledgeable and patient.
Now for the remaining two problems.
1) Win 7 still is not querying for a password before completely opening the PCL share. I checked the share in PCC / Network Sharing / Shares drives... and set it to public: no, and rebooted samba. Win7 still entered right in. I then checked from the XP machine, and got the password query. So there is a weak link in my samba settings that Win7 is exploiting.
2) Secondly, from PCL, when I try to delete a file from the Win7 share, I get an access denied regarding the "Trash" folder. I'm signed into the share using an account common to both machines, which is an administrative account on the Win7 box. I should have rights to delete. I don't know if that "Trash" error pertains to PCL or Win7 - in Win7 it should be called Recycle Bin.