My current head-banger...
Two computers (both p-mmx machines), one a desktop, the other a notebook (Thinkpad 360), both running Win95b OSR2.

Networked via SMC ISA/SMC PCMCIA 10baseT cards with crossover wired Cat5 cable (no hub).
Both units using Netbeui - no other protocols.

The desktop can see & access the notebook, but the notebook is unable to browse the network.

Have swapped in another notebook, using the aforementioned SMC PCMCIA card & all is fine. Plug the card back into the Thinkpad - unable to browse.

There's a chocolate fish for the correct answer, along with my undying gratitude.