2 Things:
If you've turned on windows networking and your PC person still can't log in, try changing the password (in system preferences/accounts) of the user he's trying to log in as. There's actually an Apple bulletin about this somewhere.
Second, you can, if you want, network 2 machines lightning-quick via firewire, including a firewire equipped PC if you're running XP. I did it just recently with my wife's laptop - very cool. You'll have to enable Firewire networking on both machines, and manually assign 2 different IP addresses, but it works great.
BTW: after you turn firewire networking back off, you may have to power-cycle all your firewire peripherals.