I don't think you're going to get this to work with MS's NFS server. The main issue (which I had to kludge around for RioBoot) is that the boot image contains several Unix-ism's such as symbolic links and device files that don't exist on Windows. So when /dev ends up empty or all the files are created but empty, it isn't tar or WinRar's fault really (it is WinZip's, though)*.

So get RioBoot and follow the instructions carefully. I've been using it for over a year on Windows 2000 and haven't had a problem.

* WinZip seems to abort when it hits a filetype it doesn't understand. If there are any good files after that point, it'll miss them. It doesn't even bother to give you an error message. That's why I included tartool in the rioboot package. If you use something else, be sure to check that you got everything.