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!

I have checked power settings for the wireless adapter and AntiVirus
software (Avast), and event logs, with no indication of where or why
discovery turns itself off.

Does anyone know why this is happening and how to stop Network Discovery and
file sharing from periodically turning itself off?

Any ideas would be appreciated.

Thanks,
Ray