ISP shouldn't need your MAC address. The MAC address stands for Media Access Control Address. It is a unique identifier for each node on a network... basically it's a hardware specific IP address for the network card in your computer that allows everything else on the network to find it.
Your cable modem has one as well which the ISP had to authorize or authenticate to connect to the network.
When you ask general questions, tough to help. So let's clear some things up.
What Operating System do both computers have?
How were you connecting to the internet before (DSL, Dialup)? With which ISP?
Your ISP might have asked for the MAC address because they want to discourage you from using a router. This means they want to charge you on a per computer/per seat basis.
Assuming both your ciomputers have NICs (network intergace cards, plug bigger than a typical phone jack) just plug it into the router. Each computer goes into a separate port in the router.
Cable modem->Cat 5/Ethernet cable->Router->Separate Cables from Router to Desktop and Laptop.
Reboot both and you should be OK assuming you have windows 2000 or windows XP.
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