Compatability problem?

I have a client who is having problems with his LAN. He is running a stand alone server with 6 computers networked from it.
The Problem: Random lockups. I am told that at random times the network freezes totally and the only way they have been able to rectify it is to reboot his server.
Systems: The server box runs 128MB Ram, 434 MhZ CPU with Win98.
4 other computers networked from this server box also run Win98. 2 of the computers run Windows XP Home and Win XP Pro.

Could there be a compatibility issue between win98 and XP?