I have an (Origo ASR-8400, if it matters) 4 port adsl usb modem router. I
used to run my XP machine from usb and my 98 machine from one of the
ethernet ports. They could both access the internet, even at the same time,
and they could also both see eachother.

But I need the usb port on my XP machine for something else. So I've
connected the XP machine to another of the ethernet ports. Both machines
can access the internet, even at the same time. But they cannot see
eachother :-(

I have a variety of ethernet patch cables, crossover and straight through.
The router came with a straight through, whereas as previous 1 port device
came with a crossover. All (4) combinations of the two cables allow both
machines to access the internet.

Given that there could be software problems at work, I wondered if anyone
could tell me which combo of ethernet cables I ought to be using, if it
matters, so that I can eliminate that part of the problem?

Thanks in advance.