XP to vista network sharing?

It took me a while to set up a network and I've been filesharing between my vista PC and XP PC, both connected to the same wireless network. (XP connected wirelessly, vista directly to router)
But recently both Vista and XP have stopped being able to access one another.
When I try to access vista through the XP comp I can see the public folder in network places, but double clicking produces "network path cannot be found" (it's a bit longer, basically suggesting I might not have permissions and must contact network admin)

How can I fix this?