Right click on your folder you want shared ( Or you can share entire drives ) , Select "Sharing" and follow the steps , You should be able to set up user level access, or in vista you can go into your network and sharing centre and allow anyone on your network to access the folder/drive.
In xp you do the same , Share the folder or the drive - set your permissions and you should be away.
Once you have done this restart both computers.
On the vista machine click "Start" then "Network" - If it has worked you should be able to see the folder/drive you shared on the xp machine in here and you will be able to access it.
In xp It should be called "my network places" - And the folder/drive shared on the vista machine should be visible and accessible.
See if you can get this to work , I have this identical setup at home with quite a few machines.
Any trouble just post again and ill keep trying to help you.