I've never used Windows Easy Transfer and I don't know how you've got the computers connected, but you can transfer files from Vista to XP with conventional networking.
If the two computers are connected, either directly by "crossover" ethernet cable or via a Network Switch/Router, then you can set any folders in XP that you want to have available over the network as "shared" by right clicking and selecting the Sharing & Security option. Once "shared" you should be able to access them from "Network" (My network places) in Vista. If you're running any third-party firewall software in XP, such as Zone Alarm, this will need to be configured to allow access to the networked computers.
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