I've got a Vista computer on which I've shared the Documents folder. For some reason I occasionally am unable to access the shares from either one of the two other computers on the network (one running Vista, the other Win 7).
The computer shows up in Start/Network, but when I click on it, it says it is unavailable, and Windows offers to diagnose the problem (but does not find a cause). Then later in the day the shares are available again. The problem is not related to startup, it happens when the computers have all been on for hours.