I'd be a little bit wary of assuming a crossover cable to the desktop would allow the laptop to connect to the Internet via the (USB?) connection from the PC to the ADSL modem. There may be a couple of things that need to be done first.
It's a lot easier with a modem-router (which could be, but doesn't have to be wireless). Straight Cat5 cables are extremely cheap (I bought a bag of 10 for £5), and modem-routers aren't exactly expensive. Then the networking etc is a doddle.