Sounds like file permission problems. I can't give you a full answer but if you look at what permissions your files are supposed to have, see how samba handles these, and then figure how to get both samba and nfs to maintain those permissions.
If its not a file permissions problem, you're SOL. You'll have to buy UNIX tools for windows to handle NFS mounts. (And no, so far as I've seen, there's no other good way in windows to mount nfs other then buying the ms unix tools)
hope this at least points you in some possibly correct directions.