Peer to Peer Networking Issue

We have two Vista computers. We keep our files on a Desktop
computer and use peer to peer networking to access them from
the laptop. Every once in a while, the desk top will not allow the
laptop to save a file it has open. And the desktop will not shut
down or restart. It will not shut down and has to be physically
powered down.

Once the desktop has rebooted everything returns to normal even
if the document is still open on the laptop.

This issue did get better with one of the updates in July, but
it still happens.

Any ideas? Microsoft did not have anything useful except to
uninstall Anti-virus to see if that was it.