I can run texthash as often as I want, it simply won't include
fxb.inc in the ls-R file.
Sorry, I'm out of ideas. fxb.inc is certainly included by texhash in
the ls-R file in our svn. I don't know anything specifically about the
texhash on Windows.
Oh, here's an idea after all: do you have any tex-related environment
variables set?