Just an update on this one.
I finally managed to get streaming working on Windows 7 (64bit). I followed the instructions in this thread below to register the .ty extensions etc.
http://tivoza.nanfo.com/forum/viewtopic.php?t=1406
With the new ozTivo image vserver is not enabled by default so you need to go and change the "NO" to a "YES" in etc\sysinit.conf (and restart Tivo). If you don't start vserver you will get an error from media player saying something about a runtime error.
Just for the record I also have the KLite codec pack installed and tried first with the Media Player Classic that comes included. This worked fine. I then tried with WMP11 (or whatever the latest version is) and it failed with some error about plugins. I tried it again later and it worked fine and has been ok ever since.