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Not sure about syncing Vista and XP files. From what I've read you can't do it with Vista sync center. I think you might have to explore a third party.
That depends on what you want to sync. Microsoft has a couple of offerings (both free) that can help: FolderShare and the new beta Live Mesh. FolderShare is quite stable, but it's limited to a certain number of files and folders. Live Mesh is awesome. It will sync almost everything. It's beta though.