I think Squashman is correct (as usual!) and you can easily test by disconnecting the cable and try to access the XP from the Vista.
On the Vista make sure in Network and Sharing Center the (ethernet) network is designated 'Private' and that file sharing is turned ON; and you probably want password protected sharing turned OFF. Also, note that Vista will not share unless at least one of the user accounts is password protected.
Then share the desired folder(s) as you would do on XP. If still have a problem accessing the folder(s), you may need to right click on the folder and start experimenting with permissions. The Help and Support Center is pretty poor in my opinion, but searching for "permissions" should turn up some helpful info.