CapnNismo wrote:

Well then I am totally out of ideas. Though I suspect the XP/Vista mix-up might be contributing.
Agreed. If it's only the XP machine that's finicky, you may want to double check the laundry list of shit-to-fix-that-MS-didn't as far as file sharing goes.

The last post may be the only help, though there is a few results in google of "vista xp, file sharing sometimes works sometimes doesnt".