You probably don't need to bother with setting up a virtual network, Virtual PC offers Shared Folders built-in.
You need to install Virtual Machine Additions, which will give you a folder (or folders) on your Mac that appears as a network share under XP.
IIRC, while running XP under Virtual PC go to the (OS X Menu for VPC) PC -> Install or Update Additions.
This will start the installer on XP, so follow the instructions on screen.
When finished (and I think you need to reboot XP), with XP running you can go to Settings on the "Virtual PC List", click Shared Folders and assign a folder on your OS X installation that will appear as a network drive under XP. It can also be configured from the Virtual PC status bar.
(Or thereabouts ... I haven't done this since installing VPC last year, and I am not sure if there are any limitations wrt XP versions, I am running XP Pro SP 2 and it works perfectly).