The easiest way to do this would be through ethernet (or firewire)
If you dont have a router, and you are just connecting the computers directly using ethernet then you need to go into network pref's and assign a manual IP address for each computer.
Computer 1:
Ip address 192.168.1.1
Subnet Mask 255.255.255.0
Computer 2:
Ip address 192.168.1.1
Subnet Mask 255.255.255.0
When you have that set up, make sure you are sharing the My Music folder and have the windows firewall disabled and then just type in the ip address of you PC by using to finder and click Go>Connect To Server.
Then type "SMB://192.168.1.1 (or whatever the ip address is of the PC)
A box should come up for authentication, just type you XP login in there and it should give you a list containing the My Music folder.
Finally just copy the entire itunes folder from the My music folder to the music folder on your mac. When you open itunes it should automatically find the songs, and you're done.