Newtork drops in and out

I have a home network between a P4 XP Pro and PIII 98SE only recently
established. This morning I was on the P4, opened a spreadsheet on the PIII
changed it and saved it back to the PIII. Now I've just tried to access it
again and the system tells me I don't have permission to access the network.
Neither PC has been switched off in the interim period and no network
settings have been changed.

The PIII 98 can access shared folders on the P4 XP but no longer the other
way around. Why is this?