For future reference, you'd likely get a better response to this in the software forum (And a moderator may move it..), but I'll try-
The first thing you should do is to connect the two computers via an ethernet cable. This should be easily do-able if you already have a home network. Just plug them both in, so they can both see the internet.
On the new Powerbook go to the control Panel, and go to Sharing. While there, click the checkbox to enable Windows Filesharing, and exit the panel.
Go back to the PC, and go t network neighborhood. From there, you should be able to naviagate into the workgroup, and name of the powerbook. (someone else might be able to provide the default workgroup name)
Open up the powerbook's icon inside network neighborhood.
The PC will ask for a username (short name) and password. Give it the username and password you set up on the new powerbook.
Then, on the PC, go to start:search, and do a search for *.mp3.
This will give you a list of all of the MP3 files on the PC.
"Select all" in this list, and then Copy and paste them into the folder on the powerbook.
This will take a while to copy. Make sure both machines are plugged in.
After this is done, you will notice a large number of MP3 files in the "Public" Folder, within your home directory, of your new Powerbook.
Select All.
Open iTunes (still on the Powerbook), and drag the files from the directory, into the iTunes window. This will let iTunes add them to it's database.
It sounds a lot harder than it is.
Good Luck!
Colin