Try this:
Open the Control Panel (icon view) and choose Network and Sharing Center. Click the link under "view your active networks" You may find it says "home network". if so, click it.
In the next window choose "Work network" that will switch you from a homegroup to a workgroup so your two computers can talk to each other.
In, and from the XP computers, right click any files you want to share on either computer and allow sharing and grant full permissions. Do this all the way up/down the explorer chain to/from the files you want shared.