I had this problem just a few minutes ago. I installed XP, installed the drivers from the OSX disc, and couldn't get online.
I checked out my drivers in the Device Manager, and noticed that the drivers (Broadcom and Marvel Ethernet) were there but had a yellow '!' beside it.
If this is you, to fix this:
Right click My Computer > Properties.
Choose the Hardware tab and click Device Manager
Click the + beside Network Adapters and Right click the Broadcom Adapter > Update Driver
With your OSX Disc In the computer:
Choose "Yes, This time only."
Choose "Install from a list or specific location."
Choose "Don't search. I will choose the driver to install."
The next area should have your Broadcom driver selected. Click Next.
You should get a screen that says "driver update successful."
Repeat these steps for the Marvel Ethernet driver and you should be good to go.