Go to your NETWORK CENTER in Control Panel & change the Network Category to PUBLIC. You should now be able to see the XP machine from Vista.
Go to USER ACCOUNTS in Control Panel > click on Manage Your Network Passwords on the left menu -- there should be an existing entry, select the properties for that entry and add a username and change the existing password.
Restart the Vista computer.
With the username and password that you provided, you should now be able to access the Vista computer from the XP machine.
This will however bypass the inbuild network security for vista, so make sure that you have software firewalls/antivirus in place etc.
Seems to work in most cases!
(yes this is a copy and paste from my last 'vista doesnt connect to my x' thread ;))