I haven't used a single Gnutella client that needed to write to the registry or windows folder...
On the same subject, FURI would be great as a very portable client... If you know how to code a web page with applets, you could embed FURI into a web page and run it from any computer connected to the network.
I found Limewire and it works.
But the downloads take much time. And most of the files are not downloadable, or you must wait a long time for the start of the transferring.