Firstly, ensure that any firewall software is configured to allow access to the networked computers and, secondly, make sure both computers are in the *SAME* Workgroup.
Then, in the XP PC, set any folders that you want to have available over the network as "shared" by right clicking and selecting the "Sharing & Security" option. Tick the box "Share this folder on the network", give it a Share Name (which is how it will appear over the network) and I would suggest, also tick the box to "Allow other network users to change my files".
Vista is a little more complicated but have a look at click here
A good "synchronisation" program is Allway Sync click here