Vista goes to sleep while shares are in use

Hi,

My Vista Ultimate machine hosts several read/write network shares. This
machine is also configured to sleep (S3) after 20 minutes of inactivity.

While I am using the network shares from across the network, the Vista
machine apparently thinks that it is idle and it goes to sleep after 20
minutes, even if I am reading or writing the shares at that very moment! Is
there a way to make the Vista machine stay awake while I am using its
shares??

I have tried setting "Media Sharing" to true, but when I do that, the
machine just plain old never goes to sleep at all, even when it is truly idle.

Thanks
Aaron