Windows Vista Easy Transfer Utility is a product that comes with all Vista machines that allows you to move user settings and profiles over to your new Vista system from your old XP or Vista PC. The utility supports both XP and Vista as sources, but only Vista can be a destination.
If you are using the Windows Vista Easy Transfer Utility, there are several things that you need to consider. Moving the settings over the network is supported, but it is time consuming and often fails. I recommend that you use an external hard drive. This takes about 20 minutes to blow onto the drive and another 20 to blow onto the new machine but it is very reliable.
Another thing to consider is that if you are transferring a user's domain account to the new PC, you will want to add the new PC to the domain first. This will allow it to retain the correct settings and will keep you from having to create a new account for that user on the new computer.