Vista - Weird Network Discovery Reset Problem!

I am using Vista Business, with a wireless connection to a private network.
I have enabled network discovery in the Network and Sharing Center, and set
the network type to private.

Here is the problem:

I can see & contact other computers on the private network with no problem -
except that periodically, for some unknown reason, Network Discovery seems
to turn itself back off. After that, when I try to connect to another system
on my private network, I get a message that "Network discovery and file
shareing are turned off. Network computers and devices are not visible.
Click to change". So I click to turn network discovery back on, then I am
able to use the network again with no problem - until the next time I need
it when it again seems to have turned itself off and I repeat the process.
This is very annoying.

Does anyone know why Network discovery & File sharing turns itself off
periodically, and how to stop it from doing that so that

discovery and sharing remain on inside a private network?

Thanks,
Ray