Thanks everybody, already. I'll try to apply your hints.
(No it doesn't work yet, I'm getting back on it tomorrow.)
bnt,
Thanks. If all else fails... etc.
Slightly O/T, but the sad joke is of course that the cable itself already linked two computers for several years... years ago.
One a Windows 3.1 for Workgroups (100MHz Pentium...), one a Windows 98 ...
Turn on one, then turn on the other (you're right about making sure one is booted up before turning on the other one), and then the partitioned hard disk of the one showed up as G: and H: on the other, and C: of the other showed up as J: on the first. Nothing else to do. But this was so long ago I can no longer remember the setup tricks, and they wouldn't really help with XP, anyway. Jeez, those "wizards"... where's Harry Potter when you need him.
At least the "doggie" confirms each recognises the other, so no hardware issue, and possibly no IP address issue either.
CJ