I'm sure all the stuff on A has addresses assigned statically, so you could just run your dsl modem into the switch that they're plugged into then run your cable to computer B. Shouldn't have any problems with it picking up inet connection from A: because it will be assigned a different gateway. Then what you'll want to do is add an alias the the nic with an ip in that same class as the computers in A. If you have any more questions, I'm pretty sure you know where you can get ahold of me ...
ADSL, sounds like the modem is an internal modem eh?
Regardless, if you want to keep internet connectivity outside the lan, you definately need a firewall. Dual NICs would work, but I wouldn't rely on XP's firewall... I'd use something a bit more secure.