Network and Sharing

I set Network Discovery to on and then select private network. It sets itself
on and all seems OK. Later I can check and it is back to off.
Several times after turning it on I have seen it go back to off after 5 or
10 seconds.
I am running Vista Home Premium on two desktops X64 and one laptop 32 bit.
This happens on the second desktop which shares files on the first desktop.
Is there some way to find out what is turning it off?
Most of the time the sharing works fine.
I do use the second desktop to run a product called VMware to run several
Linux OS's.
Any thoughts would be appreciated.
Larry.