I'm not quite sure what you're asking: how to access the Windows drive on your machine from within you Ubuntu install, or how to transfer files from your machine (with XP running) to your friend's Ubuntu machine.
For the first possibility, Ubuntu should have automatically recognized the XP partitions(s) and made them available under the /media/ directory. Transferring files should be as simple as dragging and dropping.
To transfer data across a local network (not the internet) from a Windows machine to an Ubuntu machine, you can use an application called Samba (how to install Samba on Ubuntu
.) Once Samba is up and running, you ought to be able to see any Windows machines connected to your LAN by selecting the "Network" place in the Places menu (or by manually entering smb:///
into the address bar of your file manager: